I just built the cod piece a few days ago, ran into the same issues with size/scaling. I'm using the Darkside files for mine with everything scaled to 26, even though I'm taller than 6'. I had to scale the cod up to 29 to fit comfortably (with pants underneath), but I wear 38x34s in jeans. Everybody is shaped differently so everything is custom scaled in the end...
One suggestion I can give is open the foam mark vii cod file and find the large circular piece...
(seen here:

.)
It's a circle for around your waist, but you don't even need to cut it out; Just print it and measure it with a string to find out the length, and measure your waist. Keep scaling/printing (you can even do it on the same paper) until you find a match.
You can also use the measurement tool in Pep Viewer/Designer to measure from hip-to-hip.
To find your measurement from hip-to-hip, you'll have to use the Pythagorian Theorem.. (I know, who thought math would be useful after school, right?). Measure straight out from your hip to a given point, then from that point to your other hip. That gives you A and C. A2+B2=C2; Therefore C2-A2=B2. B (not squared) will be your hip-to-hip measurement.