Do guinea pigs grow to the size of their cage?

No, they really like to interact with people. They are like toddlers, you can't take your eyes off them, even for a second! Do not use exercise balls because it causes back problems. Their backs don't curve the way hamsters and ferrets back's do, and it actually ends up hurting them even if they appear to like it. Your best bet is to use a corral (like a fence that forms a circle) on newspaper in case of accidents. They also love to be held, and are quite content to just sit on your lap and nibble fresh veggies or timothy hay. Enjoy your piggy!