If you are talking about when you sew around all four sides to yield 4 HST, then here is a chart that I hope helps you:
http://www.moonlightquilters.org/tipHSTfav.htm
Forgot to say that this table shows you the size of the actual HSTs to make the pinwheel blocks, so you still need to factor the seam you loose when you put these together. So for example, if you are piecing 4 2'' HST to make a pinwheel, your pinwheel size would be 3.5'' I believe.
Hope this is helpful to you.