I just finished a lap size cathedral window for my sister, and loved doing it! I've been thinking about the same thing, using a jelly roll. On the one I just finished, I started with an 11" square. The 2-1/2 size square would be a little too small for the 11" sq. I haven't taken the time to think about what the beginning size would need to be to work with the 2-1/2" sq from the jelly roll - but it is definitely do-able.