I am sure that someone else has better measurements on this than me.
I use the full 8-1/2" x 11" sheet for all adult sized shirts. There is obviously some difference between S and XXL, but you can't go bigger than 8-1/2" x 11". The other shirts, I just printed the design in ink saver mode on regular paper and played around until I liked it. The smallest shirts that I had were Y small (6-8). I went with a width of about 6" on those.