Here's a free calculator to help you calculate how many square feet of living space you are giving your guinea pig. Just enter the length and width of his cage in inches to calculate the square footage.
You want a minimum of 7.5 square feet for a single guinea pig, and a minimum of 10.5 square feet for two guinea pigs. 7.5 square feet is not very much space, however, and guinea pigs are awake up to 20 hours a day. That's a long time to sit in a small cage! Bigger is better, so try to get the biggest cage you can.