Try a sample using 4-1/2 inch squares - you might need to trim up the finished block a bit - but not by much.
The reasoning behind that number:
7-1/4 inch block = 29/4
29/4 / 2 = 29/8 -> which is half the width of the block
29/8 + 1/4 (for the seam allowance on one side) + 3/8 (for the triangle point) =
29/8 + 2/8 + 3/8 = 34/8 = 4-2/8 = 4-1/4
Technically, a 4-1/4 inch square should work. I prefer to cut a bit larger and cut down the HST to size.
Your assembled HST squares should measure 3-7/8 inches before sewing them into the pinwheel units. (Assuming "perfect" 1/4 inch seams and other measurements)