According to this site, the minimum is 10.5 for three and the preferred size is 13 sqft. Two of my boys are in a 2 X4 (10.5) and it is barely big enough. They still fight from time to time. My little guy, T.C has scabs all over his neck from Oreo bullying him.
Now it depends too, on the pigs. I also own two Coronet (long-haired) GP's who were raised together for three years (previous owner) in a 2ft X 3ft cage. They are temporarily in a small C & C cage (1.5 X 2.5) for now and they never fight. I am moving them to the 2 X 4 and then Oreo and T.C are getting a 2 X 2.75 (the only size that fits under the existing 2 x 4 that sits on a table). The two Coronets are much bigger than my two short-haired.
There are only 2 guinea pigs in this cage now. They do not fight and are close friends. The cage was too small for 3 boars. The 2 guinea pigs are very happy and popcorn all the time now that they are in a c&c cage.
