Many commercially made guinea pig cages are only large enough for one guinea pig, while a few are large enough for a pair. However, guinea pigs are pretty easy to contain and it is quite easy to make a good sized guinea pig home out of inexpensive materials. Here are three sites with great ideas.
Teresa Murphy provides instructions for building a great custom cage out of wire Creative Cubes panels and coroplast. Practical instructions and ideas, including finding materials and adding features and other components.
Seagull's pages feature a few ideas, including an adapted wading pool design, a home built hutch, Creative Cubes ideas, and ideas for connecting components for intricate cages.
Another cage designed from storage cubes, with a unique design. Includes detailed intstructions and cleaning advice.