The majority of cages marketed for guinea pigs are not large enough and many are terribly small. The nature of guinea pigs makes it easy to build an attractive and spacious cage quite easily, and usually at less cost than commercial cages (see Homemade Cage Ideas). The first pick here is a pre-packaged kit that allows you to put together a popular homemade cage style. The remainder are the largest commercial cages I could find, but most of them are still too small by today's standards.
These kits contain everything you need to make your own attractive Cubes and Coroplast cages. These is a very popular style of homemade cage and getting one of the kits saves the trouble of finding the components. Get the large or extra large size if at all possible. The open top design is fine for homes without dogs or cats, but instructions on adding a top made of wire shelving are included if you need a cover to keep other critters out. Payment by PayPal or special arrangement.
At 36 by 30 inches this is one of the larger guinea pig cages on the market, but it is still a bit cramped. A ramp and balcony covered with black rubber adds a bit of interest. A flip up top allows easy access. The pan is a little shallow to contain bedding.
This cage is harder to find (especially in the US) and not to be confused with the smaller Tommy 102 model. This cage is 47 inches by 23 inches (22 inches high), with a wire top that clips to a plastic bottom. Two front opening and one top opening door. There are two versions: one where the wire top is solidly welded together, and one with a collapsible wire top (which is less expensive). One of the larger commercial cages available but still not ideal.
This cage comes in a variety of sizes so be sure to get the Jumbo PR620 at 47" x 22" (37" high). You must remove the wire floor from the cage! It has a deeper plastic tray than some other cages to contain bedding, and a top and front opening door. It is unique as it is on a metal stand with casters.
This is another cage hard to find in the US but owners in other countries may have success. This may be the only commercially made cage that is suitably large for guinea pigs! At about 55" by 28" (18" tall) it would meet the needs of a pair of guinea pigs. The vendor linked here is in the UK.
This cage is 40.5" by 18" (20.5" high) with a single front opening door. Measuring the inside dimensions, you will get floor space of less than 5 square feet so it is not ideal by any stretch, but still one of the larger commercial cages you will find. I wouldn't go much smaller than this.
This cage is also about 5 square feet. The entire front opens which is a unique feature, and there is a choice of metal or plastic tray. The manufacturer's site does not specify how deep the tray is, and the cage itself is a little short at only 12 inches high.