I ran into the same issue with my niece's t-shirt quilt. She asked me to make a quilt from t-shirts that she saved from age 5 to age 19. So nothing was consistant with regard to the size of motifs I could use. After I stabilized my t-shirt squares, I picked the biggest square in the bunch and added a sashing. I measured the block and used this measurement as my target size for the rest of the blocks. I just adjusted the width or the sashing for each of the t-shirt pieces, sometimes using more than one sashing for each t-shirt piece in different colors to get to the size I needed to get to my target size. I used all kinds of colors for the sashings in all the blocks. She loved it. She used it all through college and still has it on her bed.