The size of 15 to 22" is a great size. Looking through the last swap you will see all different sizes of WHings. Fun is the name of the game.
If my quilt works out right the center will be 10x 10" plus whatever border/s I add to it. I may make each of the 25 blocks a little bigger so the quilt will be more substantial, but I don't think I need to. It is all trial and trial again!
There is also a possibility of doing attic windows for each block, or some sashing, all to be decided as I go forward.
I do start early in the time line so if I decide changes are necessary, there is time for plan B, C, D......
Have fun.
peace