I would measure the design on the shirt before deciding on a size for the block. Cut them larger than the finishing size so they can all be trimmed after the stabilizer is put on. The iron on stabilizer may have a "stretch" in one direction, be sure it stretches in the opposite direction the t-shirt does, it will be much easier to work with. Personally I love black sashing, but using the school colors may work better. I would audition them just to be sure. The small designs on some shirts can be used on the back or as cornerstones if you have enough of them.
Hope this helps.