I tried unsuccessfully to post the pictures of a t-shirt quilt that I made for my husband.
I used my 12.5"x12.5" template for all the squares. So, even tho some of the shirt designs were smaller, I just centered the template over the design and cut the nice large squares. Maybe the shirts you are working with are not big enough to do that. Mine were all L or XL mens.
I used my DH old denim workshirts for the sashings, even included the pockets and logos from the workshirts. It turned out really nice!