I was looking at the websites for the SPCA locations in my country. When I was looking at the Wellington SPCA site, I found that they had minimum cage requirements of 6.6 feet by 2.6 feet, 16 square feet! That's the size of a 2x6! Nobody can take home a rabbit or guinea pig if the living space doesn't meet their requirements. The cage must also be 1.3 feet tall. (Which is just over the height of a grid) Rats have a minimum base size of 6.25 square feet, and must have multiple levels. I'm pretty impressed by this, but obviously you don't need quite that much space for just one piggie. xD