Official calculation for HSTs is add 7/8" to the FINISHED size you are aiming for. Since most folks aren't into that 1/8" thing, the rule of thumb is cut your beginning square 1" larger than you want, then trim the resultant triangles down. Remember that your HSTs will measure 1/2" larger than your finished size. So if you want 6" FINISHED hsts, cut your squares 7", make your triangles, then trim them down to 6.5".
Pinwheel blocks are 4 HSTs, two in each direction. Decide what size FINISHED pinwheel you want, then divide that in half. This is the size of a FINISHED HST in your pinwheel, so just use the formula above.
So, for instance, you want a 6" pinwheel. Half of 6 is 3. Add 1" to the 3 and cut your starting squares 4", sew your triangles and trim to 3.5".