If he's living by himself, he'll be quite content in a cage with a bottom that measures 15 inches by 30 inches and sides about 15 inches high. The following are some key features of guinea pig cages:
Cages are generally made of a combination of plastic, metal and/or wire. Some cages are constructed entirely of wire but you should avoid these; your guinea pig could injure himself if he gets his claws or toes stuck in the wire mesh.
Your guinea pig's cage should be large enough to give him ample room to move about. The recommended cage dimensions for one average-sized guinea pig are 15 inches by 30 inches, with a height of 15 inches, but he would make good use of a cage twice that size and be happy to have the extra room. Of course, if more than one guinea pig will occupy the cage it will need to be larger.
The floor of your guinea pig's cage should be solid - made of plastic or plastic-covered wood. Some cages are made with a wire floor so droppings and urine can collect in a tray beneath the cage floor. But the wire mesh may eventually hurt your guinea pig's feet; for the sake of your pet's comfort, provide him with a piece of wood or carpet that he can rest on.
Cages with plastic-slat flooring are designed along the same principle as wire-floored cages. These types of cages are more comfortable for your guinea pig as well as easier to clean. Be sure, though, that the spaces between the slats are small enough that your guinea pig's feet won't get caught in them.
Wire- and slat-cage floors usually contain a plastic tray underneath to catch urine and droppings. This tray can be either the pull-out or the snap-on variety. The pull-out type of tray will be more convenient for you; with the snap-on trays, you have to lift the cage off at cleaning time.
The door of your guinea pig's cage should be wide enough to allow for easy access. Preferably, the entire top of the cage should lift up, with doors opening outward. If your guinea pig won't be bothered by other pets, you might not even need a top on the cage because guinea pigs do not jump or climb. For safety's sake, you should check your guinea pig's cage periodically for sharp edges that could pose a hazard. Also keep an eye out for any broken wires that could injure your guinea pig.
Several items are indispensable to the well-appointed guinea pig cage. These include: