Originally Posted by MTS

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. ;-)