I'm not an expert (one will probably be along soon!) but I have had 4 ladies living in a 5x3 C&C (actually its 5x2 at one end to fit round in an alcove) for 3 weeks now with no problems. I think the cage size guide says to assume 4x2 is ok for 2 ladies then add an extra 1x2 (2 square feet) for each extra piggy?
But now we have added a little husboar I've ordered an extension that we'll be adding on this week to make a 5x5x2 L-shape!
Might work differently if you're adding piggies into an existing set up though, mine all moved in to the C&C at the same time after being kept as pairs in Ferplast 120 type cages so they were really impressed with the extra floor space!