RE: what size load....
You can shoot a wide range of shells, the real issue is what you are used to and how long the shot is. I have stuck to #6s over the years, and never change. A few times I switched to 7.5s in the winter when the leaves fall off the trees, but it didn't work for me. I wounded a few because I wasn't great a judging the range - and that extra 10 yards is the difference between a wounded squirrel and a dead one. Some folks I know use 5s, which gives you a pretty long range, but in my experience 5s break too many bones.
I use No. 6s and pick my shots carefully. If they come out of a hole that is 10 yards away, I wait until they are 25-30 yards to shoot. I generally shoot a few dozen a year, and most years I use one shot per squirrel.