It really depends on the pigs and the height of the cage. If the pigs are quite good jumpers I would keep it around 30cm. If not, then you could probably go into the 20s if they're not too bothered about getting out of their cage. Also, if you are thinking of raising the cage up I doubt they would try and jump out. My piggies are in a 35cm high cage and they never even put their paws up on the bars so I seriously doubt they would jump out!