Are 'human' models the only option you have for the 'giant' characters?
Reading through the description, I understand that this story takes place in a dystopian future in which giant aliens have taken control of Earth. If this is the case, it would be cool if the alien looked somewhat...well alien. :) (Or if there was an option to have alien giants)
I'd highly recommend female Quarian's from Mass Effect, or female Spartan's from Halo, or Captain Phasma from Star Wars. (I know the last 2 aren't aliens, but I really like their armour)
I may be talking complete nonsense here as I really don't understand what would be required to port these models over, but again its just an idea from perspective.