My friend and I just got through building the chest pieces 1-2 weeks ago. Everything is scaled perfectly. Here's what you need to do:
Get online and find a bunch of reference pictures. There are hundreds around here. Find simple pictures that give you the best idea of the dimensions of the piece on the character's body. For the chest, go off of the width. Look to see where the main chest piece extends. Pick those same two points on your own body and measure the width - convert inches to millimeters (1 in. = 2.54 cm = 25.4mm). Then input that same width value for the pepakura piece and build it. Make sure it looks right, and if it does, then size the rest of the pieces off of that confirmed-accurate piece. By that, I mean find reference points from each piece as it comes together and measure (ie: the top of this piece to the middle of that piece). It should come together nicely as long as you just have good reference pictures and an eye for reference points.
And like they said ^^^, there are tons of stickies that you should use if you have any other questions. I figured I'd throw in my two cents of advice given the very recent success I've had, and hopefully help you out. In my opinion, this is the best way to scale the pieces - each part individually off of reference points transposed from the character to your body.
