7.5 square feet is what I'd consider the barest minimum for a pair, and closer to 10 is more ideal.
I really like the cube cages, and all my piggies live in them. However, if I couldn't build one, I'd buy the Midwest Guinea Pig Habitat, because it actually is an ok size for pigs unlike the ones that Petsmart sells.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry