Another option is to find a fabric you like as "sashing" and buy enough to make an entire quilt top (like making a backing). Then applique the t-shirt blocks to the top in whatever layout you like. The blocks automatically get the same "sashing" and you don't have to struggle with differing block sizes. I've done this when the shirt blocks were hugely different in sizes. It saves lots of headaches, you don't have to add any sashing, and finishes nicely.