I bought the lightest weight black iron on interfacing. It cost me about $28 since it is only 22" wide and my t shirt blocks were 15". That figure is based on cutting out the largest logo in the batch, squared up with at least 3/4" around the words/logos. I over cut them to start, after first cutting off sleeves, neck, and back. Then I measure the largest to establish block size and know that all the rest have to be that size. Then I cut the interfacing to fit the oversized t shirt block. Be sure and first press each t shirt over cut piece on the back side. If the front has a rubbery logo, I try to avoid using it. If it is really special and they want it. I cover it with a pressing cloth, so it doesn't mess up your iron. Then I re-cut the fronts to the right size (there is no "right size", just so that the largest one is considered and they all match). Even with the lightest weight interfacing, these squares become quite heavy. Make sure that the interfacing has a really good bond!! Especially if you are going to have a long armer quilt it. They won't do one if they are not smooth. I used 3/8" seams so they are not so stiff, when I added the 3" sashing. Hope this helps you when you're ready.