Thanks for all the helpful advice so far. While I do like the look of sashing on some quilts, I really wanted a "blocking" scheme for this and I had already bought ($$$$)
my filler fabrics. I do have the book the pictures I linked to above came from. The author does explain how she achieved that look but her t-shirts were all cut in multiples of 2 plus the 1/2 seam allowance. That's why the layout looks so nice with block infill. I think what you are saying yel is similar to what I just decided is the best course of action - sew a border around each t-shirt until they are multiples of 2. Then they will fit into the grid much easier and extra space can be added with squares, rectangles, etc. Now, when I add these borders, I do of course need to add the seam allowance, corrrect? So for instance, the Mickey t-shirt is 11.5 x 13, so I'd add either an inch to the 11.5 side to get 12 inches with the seam allowance, or inches to get 14 inches aftr seam allowance; and an inch and half to the 13 inch side to get a 14 inch block with seam allowance? Oh man my head is spinning.
Edit: And maybe no added border on the ones that are already multiples of 2, like El Paso which is 10 x 12. But that doesn't take in account the seam allowance.....soooo