Zombie's usually pretty good with the scale, but really it's a matter of personal preference to the size, how long you want it to be. The pep program gives scale in mm. 300mm is just shy of 12 inches.
As for the armor itself, because it's "soft" armor (leather, cloth, etc.) the traditional pepakura technique wouldn't work very well. Probably would be better to search for bodysuit tutorials, or just buy one if you're not too confident in your sewing skills (among other things they've been called bodysuits, catsuits, or zentai suits), and then add the details to it using reference screenshots. It is my understanding that of it can't realistically be made from fiberglass or foam, Zombie is skipping over pretty much anything that isn't hard armor, stuff like leather, hide, fur, and any portions of the armor sets that would be made of such materials. Armor sets like the Thieves Guild, Shrouded/Dark Brotherhood, and Nightingale armor would all be fairly difficult to translate into pepakura.