Sandy anysize between 12.5 and 24.5 is fine. Each round will be 4.5 inchs or 6.5 inchs so it just has to be sized so one can put a block of either of those two sizes on their round if they want to do a bunch of blocks ie pinwheels or such. if we end up with an off size it makes it harder for the next person to do their round without a whole bunch of math. I do not do a whole bunch of math lol I get all flustered trying to readjust. Those look like beautiful blocks. Remember the larger the block you send the larger your quilt will end up so its up to you what your hoping for. and your block does not have to be square it can be 18.5 x 24.5 if you want it to be so use the basic sizes: 12.5, 18.5 or 24.5 for your starter and the rest will all work out fine. A red and white quilt.. and Jaba has a beautiful purple and white quilt.. what will the brat do.. might be a good time to pull out my turkey fabric on you all or perhaps my skunks.. no? well heck time to go see what I have on my shelves. some gorgeous aqua metallic Kauffman fabric might say make me make me!! up for some blues ladies?