I started with 5" blocks...sewed together comes to 14".
(5 + 5 + 5 = 15" less seam allowance of 1/2 & 1/2= 14)
Cut that 14" block into your 4 sections...now each mini block is a 7 x 7 block.
Square those to 6 1/2" (leave your "littlest" block in corner on each one you trim) and you'll have it nice and even.
This will sew to a 12 1/2" block
(6 1/2 + 6 1/2 = 13 less 1/2 seam allowance = 12 1/2)
Otherwise you have to deal with 1/8" measurements and I don't do those well :)