Oh nice! You (if you don't already know about them) should check out the 405th.
here They are THE videogame (mostly Halo universe stuff, but other franchises like Mass Effect too.) costuming group. If you have questions or want inspiration etc. that's the place to go! Also tons of great resources like Pepakura files and casts.
Some examples of members builds:
Halo 4 MC Helmet 100% accurate, must have taken insanely long.
And yeah, that's a real full sized build of Kat's Armor... creator is wearing it... lol looks like a scale model or toy! She even did the fake robotic right arm.