If you go with making a C&C cage, make sure there's some mesh or corrugated plastic to cover at least half the height of the cube! My sister and I just constructed a cage from cubes for our new piggies, and the first thing Merlin did was squeeze out through the holes. -___-; He and his brother are both about 8 weeks old, so they're palm sized.
When I had rabbits, I was told the rule of thumb for space is there should be at least enough space for 4 times each animal's body size in the cage. At minimum.