I know! I know! "It depends"! But that isn't helpful!
Not asking how much of a specific crop, i'm asking how much space would it take to grow 100% of all food for a vegetarian family.
Problem with google searching is that they seldom specify whether the numbers they give are of how much space it would take to grow all of the years produce in one sowing or if they are replanting in the same sport after the first crop is harvested. In Mild climates like my dear PNW, we can harvest year round and grow 6-7 months most years. For instance, in the very same article they estimated 4,000 sq feet for ALL a persons food (staples and vegetables), 1000 sq feet per person for vegetables and 2,500 sq feet per person for vegetables. I don't know why the author cant be consistent as to how many square feet one needs for thier vegetables but I suspect that 1000 is replanting the same bed after harvesting and 2,500 is not replanting. What do you all think of the 4,000 sq feet for all food estimate? I read elsewhere that one needs 10,000 sq feet per person for staples but in the PNW we can easily get two, maybe 3 harvests of staples or a combination of staples/vegetable harvests in the same space (ex: quinoa sown in March followed by beans in June followed by Winter rye in September).
Since, of course "it depend", lets say: PNW Washington, intermediately ideal location in regards to soil quality and sunlight, fertilized with manure compost, human urine, and cover crops. Minimum irrigation (as in not intensively spaced). a variety of vegetables (high yielding and low yielding). Staples including: squash, pumpkin, overwintered potatoes, quinoa, rye and overwintered field beans (again, high yielding and low yielding). Heirloom seeds (not ultra-high yielding hybrids that can't stand weed pressure).
I'd really just like a ballpark estimate. Maybe even an "in my experience, though I don't live in the PNW, my growing season is only long enough to get one harvest of staples and I need (XXXX) square feet of (crop), (crop) and (crop) to feed my family through the year. Furthermore, i need (XXXX) square feet of vegetables which I can start planting in (month) and harvest until (month).
Since you started off with so many disclaimers, you already realize that the question isn't really the right one to be asking.
Let me answer it not as a gardener, but as a forager. Most of the time I forage for food I do it from inside my fence lines (my garden), but I'm not adverse to gathering a harvest from elsewhere, when the occasion arises. I grow no kudzu of my own, but I know of plenty of places around town where I can stop and fill up a bag's worth. I have a loquat tree that is too young to be producing, but the two big ones at the library down the street usually have a bounty in the springtime that goes unharvested. I can forage there, bring home a harvest, and that's less "garden space per person" out of my yard.
And then there is the matter of trading. My neighbor has a bunch of pecan trees and always has a supply of pecans on hand. However, he doesn't grow any turnips, which I do, and he is always willing to swap pecans for turnips. How does that affect the calculation of "garden space per person"?
I think the "garden space per person" question comes from a factory mindset, thinking that for a certain size factory, we get a certain amount of output. How about we alter the question more along the lines of what a field biologist would ask, "how much foraging territory per person?" That's a question that is easier to measure, and it works for both carnivores and herbivores. Tigers have a huge territory, something like 25 square miles, because they have to range over quite a large distance to find a nice size animal to take down for a meal. Even if you ask "how much space does a hunter gatherer tribe need", it depends greatly on the productivity of the ecosystem. Plains Indians covered vast distances as they followed the herds of buffalo, but tribes in California were much more sedentary, especially along the coast, where there was always something to eat in the local tide pools.
Foraging is also more in keeping with the idea of a forest garden. My property is not a forest garden yet, maybe on its way to being there, but I look upon my garden not as a factory where I can collect harvests and more as a place where I have optimized the foraging so that I can always find something to cut and take into the kitchen when I'm hungry. Some crops you really do have to do a formal harvest and then prepare them for long term storage (corn, peas, peanuts, sweet potatoes, etc.), but there are many others that are ready to be harvested any time you are (onions, hot peppers, leafy greens, taro, herbs, etc.)
If you are in a climate with a year-round (or close to) growing season, you can come close to that continual harvest, and then the amount of garden space required to support a person begins to approach the biomass productivity of the land. If you are on a tropical Pacific island and live on a diet of taro, coconuts, and bananas, you probably don't need much garden space at all and an acre of land could support 10 to 20 people. But humans like variety in their diet, and even if they could get all their requirement from a small plot of land, they are much more likely to think of the whole island as their foraging territory and they will know what's in season and when and where to find it.
There was a guy who tried it here in Portland, OR, who was on one of Paul's videos. He had about 1/5 acre. He couldn't make it but he didn't really grow fruit. I think you would need to grow fruit. I realize that I disagree with Paul Wheaton on this one, but I think you would need to graft different varieties on your trees, some for storage fruit. Spread out the harvest time. Many apples will keep until May or June, when your strawberries, honeyberries and salmonberries are starting to come in. Some trees like quince and Asian pear are amazingly productive. Kiwis also store very well. Fruit also takes advantage of the 3 dimensionality of the land, and it diversifies well with vegetables and edible weeds. Nature uses the 3 dimensionality of the land to control pests: spiders, hummingbirds, and all of the helpful insects that pollinate and control the pests.
I agree with John E that it makes more sense and is just a lot more fun to share, experiment and forage. Even recipes. Also, some things like mango, papaya, coconut, guava are impractical to try to grow here.
Dan, it really does depend upon.....( fill in the blank, there's lots of possibilities). Here's some suggestions for the blank...
.....which vegetables your family eats. For example, beans are pretty productive if harvested for green snap beans, but if your family wants lots of dry beans, then you would have to devote a lot more square footage for that. If they prefer summer squash over winter squash, then not as much square footage is needed. Many winter squashes need large areas.
.....if your family wants corn. Corn takes a lot more space and time.
.....if you're expecting on producing all your grains. Grains will tie up a lot of square footage.
.....if your vine crops grow on the ground or up on trellises. Vertical growing takes less space, but in some cases the plants are more productive growing on the ground, which of course uses more square footage.
.....if your crops will be sown in rows or wide beds. Single rows will use up more space, but they take less man hours to maintain them. So if you are planning on doing all the work yourself, then rows with their higher square footage requirement is what you'll be looking at. Unless you plan to go with permanent deep mulch. I don't know if that would work in your area. When I initially set up my homestead gardens I found that I could maintain several quite large gardens by myself using a mantis tiller as long as I used rows. Over the years I've gradually switched to wide beds as I got the ground improved and a better system for producing a lot of mulch. I still need a small tiller for quick bed preparation between crops. No-till doesn't work for me for my annual veggies because I need to produce a lot of food with reasonably minimal labor and time.
....if you are good at intercropping and succession crops. I've gotten to know which crops I can tuck in here and there that won't interfere with another crop. For example, radishes, turnips, and bok choy can be planted between baby tomato plants because they will be harvested before the tomatoes get big enough to shade them out. By growing daikon in the spring, they will come out just at the time the soybeans are ready to sow.
There are probably more things that could fill in the blank, but these are the ones that came to mind first.
It's never too late to start! I retired to homestead on the slopes of Mauna Loa, an active volcano. I relate snippets of my endeavor on my blog : www.kaufarmer.blogspot.com
As the above replies have stated, it all depends on so many factors that the question is nearly impossible to answer.
I would suggest looking around for a book called 'Paradise Lot'. Two guys in a Massachusetts suburb (If I remember correctly) are doing serious stuff on 1/10 acre lot. You can also find some videos online to get a more visual idea of what they are doing. An excellent reference book and a real eye-opener (at least for me).
One thing that I took particular interest in (and it goes along with what John E said above) was the way they have a couple of banana trees growing out in front of the house. Obviously, you aren't going to get a crop of bananas in Massachusetts. But the guys call them "pork trees". The neighborhood is home to people of Caribbean and South American heritage. The neighbors come and take leaves from the banana trees to make traditional dishes. In return, they drop off some of what they use the leaves to make. You can't raise pigs in the space it takes to grow a banana tree, but people will bring you home cooked pork wrapped in banana leaves if you let them take a few leaves off your trees.