Go to 2D menu > Change Scale. A little box will come with 4 measurements: Height, Width, Depth and Scale Factor.
To change the size of a specific part you simply change the numbers.
I would start with the chest piece, roughly measure from your shoulder to just below your rib cage, this will be it's height, you can then do the same for the width but you shouldn't need too unless your stick thin or on the larger side.
Print out the chest piece, make it and then see how it fits. Make any adjustments to the Pep file if it's not right.
Once it's the right size you'll want to open up scale menu again and jot down the Scale Factor (it'll be a long number like "1,23456789").
Now use this number to scale the back and shoulder portions.
Repeat the whole process for the leg pieces
#Don't just use the same scale factor as the other parts#
It's all a bit trial and error when it comes to scaling, the original 3D file could have been made without sizing properly.
Hope this helps

Oh and make sure your paper settings are all correct, it defaults to US paper sizes.