I do my T-shirt quilts differently than most people. I do not use a set block size. Instead the blocks are in 4 inch increments (plus the seam allowance) based on the design on each T-shirt. So I would have 4 x 4, 4x8, 4x12, 4x16, 8x8, 8x12, 8x16, etc. With the largest block being 16 x 16. This is the method used by Too Cool T-hsirt quilts.
I typically do not use stabilizer when making T-shirt quilts - only if the T-shirt itself is very flimsy. I don't use sashing between the blocks but I do usually put a regular quilting cotton border on the quilt and use quilting cotton for the backing. I have made 6 or 7 of these now. All but one of them was twin size or larger. I machine quilt on my domestic sewing machine. Here is a link to one that I made.
http://www.quiltingboard.com/main-f1...ml#post4790296