Several years ago I had a Winchester Model 12 in 20 gauge with a full choke barrel. My reloaded shells with 7/8 or 1 ounce of #5 shot was great squirrel medicine. The squirrels usually dropped like a rock at any reasonable shooting distance. And, it was easier to find the larger #5 shot when cleaning the squirrels. Typically I aimed for the head (especially at relatively close ranges) to minimize shot in the meatier parts of the squirrel.

It is difficult to find commercial 20 gauge shells with #5 lead shot. I personally would not use any shot size smaller than #6 on squirrels, regardless of the gauge.

Also, I would suggest shells loaded with typical lead shot rather than some of the "non-toxic" hard shot used in many shells specifically designed for turkey and waterfowl. Your teeth may thank you.