I have to respectfully disagree entirely with the math in that whipup link above. The relationship is NOT proportional as the blogger states.
The math would be the same as for QST's. You're just sewing the first seam differently to get to the same result. For any finished size HST, add 1.25" and you have the size of the squares to start with. Your finished pinwheel will be twice the size of the finished HSTs, so for 4" finished pinwheels you need 2" finished HST's, which requires two 3.25" squares.