If you want the unfinished block to end up 12.5 inches square after you've cut it and resewn it, this is what will work:
I've attached the drawing. It does require three different sized pieces.
If you want to make all the squares the same size, 4-2/3 inch squares will work. 4.67 inches.
5/8 = 0.625
3/4 = 0.725