If you want a finished size of 12 inches - and assuming 1/4 inch seams -
Cut the four corner pieces 4-1/2 x 4-1/2
Cut the four side pieces 4-1/2 x 5
Cut the center piece 5 x 5
After the block is seen, cut, and re-assembled - it should measure 12-1/2 x 12-1/2 unfinished - 12 x 12 finished.
Actually, the ratios can be changed to alter the look of the block.