How big does a guinea pig cage have to be?

I have a female, adult guinea pig and I am planning to build a C & C cage for her. Store bought cages always seem to be a bit flimsy and too small. What is the minimum cage size for 1 guinea pig? Any tips, links, instructions, or material lists (please mention where to buy the items) would be greatly appreciated! I really don't have any experience with this type of stuff, so I could use some help! :P Thanks SO much in advance! :)