Just measure the length of the body part you are making. The width and depth stay proportional to the height in Pepakura to keep the piece proportioned (obviously). The fact is, you are not as muscular as a Spartan. None of us are. As long as the length of your piece is right, you can bulk up with your undersuit and padding.
What I suggest, and how I did it, is build one whole side of the suit first (the pieces there are two of). Then, once you know the right gauntlet, for instance, fits, you know what size to make the left and you haven't made TWO pieces that don't fit.
Good luck, man, hope that helps.