6 1/2' to start for the yellow squares and 3 1/2" EACH for the pinwheel block.
When you sew the pinwheel blocks together you have 3.5+3.5 = 7 -.5 (seam allowance) = 6.5 Do that twice for each square.
To assemble: 6.5 (pinwheel block)+ 6.5 (plain block) = 13 - .5 (seam allowance) = 12.5