When I did one like that, I used graph paper to help figure it out. You have the right idea of picking a finished size. You just need to know how much to add to each piece to get there. Graphing it out will help. Another thing that might help is to use the graph paper and cut out pieces to scale of the tee shirt sections and on another sheet, draw a grid of the finished size quilt and block outlines. Then you can move the little pieces around and get a feel for what it might look like different ways. You can add borders all around the tee shirt piece, or use it to make a block that comes out to the finished size you need. I did a border all around. On some, I had cut the fabric bigger than I needed and squared the blocks up to the same size after.