Questions about apple trees

Background:  I purchased a farm about 10+ years ago.  It was a farm for cows since the 1970s and the farm has been operational since 1750.  I am unsure what the farm was used for prior to 1970 / previous owner.  On this farm are about 50-100 apple trees that are scattered about in the many different fields, along the tree lines of the land, and a few in the woods.  There are apples that are sweet, apples that are tart,  at least 30 or more different unique flavored genus of apples with colors like red, dark red, green, golden/yellow, and striated.  These trees are upwards of 30 to 50 feet tall and are wild. I am in Zone 4,  roughly 100 miles south of Canada. There are limbs that are clearly dead on many of the trees.  

Problem:  These apple trees appear to have never been maintained and the fruit just dropped so cows, horses and deer could eat them off the ground. Fruit is small compared to the trees I persosnally planted;  perhaps half the size of the new trees I bought from Pams Trees about 6 or 7 years ago.

Questions:  How do I prune these trees for their health?  How do I prune these trees so the fruit becomes bigger, like store sized apples /  the apples from my new-modern trees?  When do I prune /  do I prune in spring, summer, and fall?  Or just at a specific time of year?  

Note: I do own a few chainsaws,  I have a medium size (50 horsepower) Deere with front bucket and can get about 20+ feet high.  

First they may be small sized apples since they are wild trees, not even half of the known species are large (red delicious sized). Pruning starts with dead wood, next is crossing or rubbing branches. Give the freshly pruned trees a 2" thick layer of compost mulch, this should provide new nutrients to the roots, which will help the fruit grow to it's proper size.

Redhawk

Also do not prune more than 20% off a tree in any given year (dead wood doesn't count) You'll probably find that no matter what you do the apples stay small. But i have to say, what a lovely problem to have!

I would prune now before spring.
The trees are probably just neglected, not properly "wild" apples so I guess with pruning you could stimulate new growth and get bigger apples in the end. There are methods that are quite radical (I don't know the english term, in German it is "die Krone abwerfen" or toss the crown, which you usually do before grafting). You will sure find something when googling.

Apart from the points given by Bryant, make sure that you open up the crown enough to get more light into the tree.
You could start with a few trees. I personally think tree pruning is tedious and takes a lot of effort (and you have to have a ladder and/or climb into the tree), so not one of my favourite tasks. I do most pruning intuitively and our tree had so many apples last fall that we are still eating from them!

Googling brought me a little video on pruning a 50 year old tree. It is all in German so you can just quickly compare the tree in the beginning and in the end after pruning: http://streuobst.naturpark-suedeifel.de/2-baumschnitt/sanierungs-oder-pflegeschnitt-als-rettungsmassnahme

Another site suggests that you don't completely remove dying trees but just stabilize them enough (against storms) because they are habitat of numerous species (owls, dormouse, woodpecker etc. according to your wildlife).

Prune while dormant. There are open vase and central leader styles in terms of shaping but when you have older neglected trees (I have 2) first I'd just focus on removing deadwood, crossing branches, and damaged wood. Basically open them up a bit.

Michael Phillips "Holistic Orcharding" book is my go to reference. He has a lot of info on timing for management tactics and identifying disease and pest problems.

Good luck! It's definitely a project. I'm 3 years into renovating the old trees we have and so far one has been stubborn about bearing while the other tends to go bi-annual though I do get some apples now in the off year.

*photo from when a storm took out a big limb on one of the apple trees... geese enjoyed the premature apples.*