really depends on the drill and the distance, but as a rough guidline, I use 3x5 cards from 7-15 yards. I orient them either vertically or horizontally, as the drill dictates, and then make sure that I can see what I need to see in terms of recoil on the target. I haven't done a lot of intermediate distance stuff, but at 100 yards, I go back to 8" steel. A buddy of mine writes for SWAT under a pseudonym, and he has written much more extensively about target size and sub caliber trainers. He has a formula for determining target size and distance, but I hate math.
Incidentally, I have put over 1000 rds of fgmm .22 through my Tacsol, in one weekend, and not had any function issues. Similar experience with one of my Smiths. Both are very accurate, but the Tacsol has a slight edge over the smith's I have.