The adequate space for the cavy is at least 1 ft by 2 ft per guinea pig, the bigger the better. The cage should be easily cleanable and the cage and materials it is made out of must be easy to clean and disinfect.
A good guinea pig cage should provide adequate air-flow as cavies can develop respiratory problems if forced to live in stale or ammonia-saturated air (from urine). It should be also safe to protect the cavy from other household pets.
Last but not least - your guinea pig cage MUST have a solid floor to it - cavies do not have thick foot pads nor do they have furred feet, such as rabbits, and they cannot tolerate wire flooring well.