firstly wich cod piece for what armour ?
If its for the MkVi what I did was measure from my groin up to just above my belly button then put in that height. You may have to add or remove some of the belt piece depending on your build.
This is pretty easy to do ...just glass the parts and cut in two then try it on if its to small simply cut a few pieces of card stock to shape and attach them ..then test fit again.
Once your happy glass the extra pieces of card stock