Mine is in the basement and about 6' wide by 4' deep with the door in the middle of the 6' side. It's poured cement against the middle of the North wall (I didn't build it otherwise it would be in the NE corner). I insulated it from the interior of the house with 4" of styrofoam and have two 4" pipes going in and out of it through the rim joist. The floor is cement.
I removed the decades-old
wood shelves this year and put in plastic shelving. Much better (other than the plastic part) for cleanliness and mold prevention. I also lime washed it which made a huge difference in how clean it looks/feels and how moldy I expect it to get.
For me and the missus and a huge garden, it's big
enough. I store apples, carrots and beets in 5 gallon buckets in dry/damp/damp (respectively) planer shavings. That's because the cellar isn't damp enough for the root crops. I have onions and potatoes in baskets on the shelves and they do fine till spring.
I love how it's in the basement and easy to access. I hate how it's in the basement and takes till November to cool down. I love how it's in the basement and it barely gets too cold after a stretch of -10F weather and I can fix that by cracking the door open. I hate how it's in the basement and can't get humid enough. I love how it's in the basement and easy to access (yes, that's in there twice on purpose).
If I were doing a new one in a new house, I'd bump out the basement wall for the cellar and have a gravel floor. I'd try to figure out how deep undergound to put that part of the foundation so that the cold outside air optimally cools the cellar. My foundation wall sticks out of the ground a foot and has an inch of styrofoam outside it. With that amount exposed to the root cellar's back wall, I almost freeze the cellar on cold snaps in January in Northern WI. If I was farther south, I'd probably want more wall exposure to the cold outside air (I think).
I'd also build it more like 6' by 6' for the two of us. It's currently big enough but a bit more space would
be nice.