You will need to make your half square triangles so they end up being 2-1/2 inches after they are sewn. The full blocks in the quilt would be 2-1/2 inches. The block is a 9 patch. Do you have the pattern or did it come off a website? There are several tutorials here on the board that tell how to make the HSTs as everyone has their favorite way. Hope my explanation doesn't confuse you more. I am a visual person where trying to explain things.
mltquilt