Measurements for heads are across the top, from ear to ear, and around the head at the forehead. From that, you can scale everything else. If you compare the difference between your measurements and the measurements in pepakura, you can adjust the scaling by the percentage different. I recommend adding 1/2 to 1" to the diameter for padding, to ensure a proper fit.
I may not know costuming well, but I sure know armour.