Oh we had guinea pigs for the kids - they're fab wee beasties
The hutch we had was a two story one, the top had an open area and sleeping area and a long ramp ro the run below. Sleeping area was about 18" square, likewise the top open area and the run was about 4ft square, but you could easily have added an extension. We moved it every week or so to give them fresh grass and brought them in to a big plastic/wire indoor cage in winter (Scotland)
They've got bags of character and are quite chatty