When selecting a cage for pet guinea pigs, it's important to consider the size of the cage based on the number of guinea pigs you have. The recommended minimum size for one guinea pig is about 7.5 square feet of living space, but it's best to provide more space if possible. For two guinea pigs, the minimum cage size should be around 10.5 square feet. Additionally, make sure the cage has a solid bottom (not wire) to protect their sensitive feet, and include hiding spots and enrichment items for their mental well-being.