21" FINISHED divided by 1.414 = 14.85" + .5" seam allowance.
Rounded up to 15.5" CUT size.
But I'd cut them bigger (16.5+) to have the center float - gives you more wiggle room.
Double check it the other way to find the diagonal of a square:
15.5" square - .5" seam allowance = 15" FINISHED x 1.414 = 21.21"
Close enough. ;-)