Not too big. All depends on the load and where you are hunting. I use 12, 20 and .410. The 12s will give you a bit of reach, especially if hunting in early fall when there are still leaves on the trees or in pines like I do. It also gives you some extra firepower if you happen to hunt where you may also run across birds or foxes and such. I load up with #7 or #8 in 2 3/4" field loads and carry a few 2 3/4" or 3" heavy shells in #6 for extra punch.
Maybe a couple of 3" #4 or 4 buck if there is agood chance for coon or fox.

I don't have any books or websites off hand, I leaned off of my father myself. But the best advice I can give you is to dress out and skin them as soon as possible. Once they cool down they can be a real bugger toskin. It's a lot easier to do fresh. I do mine as soon as I kill them. Carry all that I needright in a field pack set up specifically for squirrel hunting, including heavy duty ziplock bags for the skins andmeat, and some waterless hand cleaner and small junk towels for cleaning up. Got the bag at Walmart (gasp!) It's one of those camo shoulder bags they sell, just the right size to fill up with everything needed for a specific hunt. Got one for small game, deer, archery, muzzleloading, etc. Keeps everything I need in one handy "go bag". then again, you end up having X number of knives, etc to fill them, but I already have a thing with "collecting" gear anyway.

De Nada.
It's what "we" are here for.