If you are using 4-HST to make the pinwheel, you will want 2-3 7/8 light and 2- 3 7/8 dark squares to complete 4 HST blocks and equal 1-6 1/2 unfinished pinwheel
A lot of us do better with cutting 4-inch squares and then trimming the sewn HST's down to 3-1/2 inches. Usually it's just a matter of neatening up the sewn HSTs/