If you are wanting to make a 9 inch FINISHED pinwheel block....
each HST should be 4.5" finished (5" unfinished) if you are making your HST by using two squares drawing a line corner to corner and sewing a 1/4" on either side then cutting along the line you would cut your squares at 5 7/8" OR cut them at 6" and trim after you are done cutting them apart!