I liked the fusible tricot interfacing. You just have to be careful to put the stretchy direction opposite the way the tee shirts stretch. It's nice and soft though, not stiff at all and did adhere well.
As for the sizes, I ran into that also. I wrote down the minimum measurement to get the design on all the tee shirts, the I decided how big to make the block based on the largest one with a small border. I added whatever size borders I needed on each section to get them to the size I needed. I used various colors of borders, and tied them all in with a colorful wiggly stripe fabric. It came out great.