This uses more than just pep, but another way to somewhat scale things is to use a picture of yourself as straight on as possible from both front and side, maybe back or more sides as well, ideally in the same pose, and put those on image emptys in blender. You can then position and scale those to each other like they were taken. Then, you can scale the parts to those pictures and check if they might overlap with you using plane cuts on different axis. Once you're happy with the scale, you can use the ruler tool (CTRL to snap to verticies) and some measurement on the same body part and calculate the scale for your parts. I'd recommend to this whole process again with new pictures and ideally the other armor parts in those for different parts as the pictures aren't orthogonal and the scale might be off a bit more. This is basically poor mans armorsmith if you don't have a 3D model of yourself. I'd recommend to give yourself enough room so you don't make your things too small, as this isn't the most accurate method. One downside is that you need the model in a format that blender can read, so you either need the paid version of pepakura or someone who can convert it for you. If you have neither of those, armorsmith is cheaper than pepakura.