I reread the original post & realized you wanted info on how to cut the triangles for different pinwheel sizes. FYI... I typed in Pinwheel Tutorial in the search box and had 8 pages of answers. This is the first one I saw that could answer your question.
http://www.quiltingboard.com/main-f1...p-t212644.html
The following post gave several websites that will give you more info about sizes & different techniques for HST's.
http://www.quiltingboard.com/main-f1...l-t211236.html
Too funny....I had always wondered about the measurement too. Glad you made me look for the answer. Have a great day sewing!