As for size, a rule of thumb is a minimum of two square feet per guinea pig. If they are not allowed to run around for exercise on a more or less daily basis, they will need more space to be happy and healthy. The main thing to remember about housing cavies is that these little animals need to keep dry. Damp conditions combined with cold soon bring on illnesses such as pneumonia and this results in an early death. Cavies can cope with cold conditions as long as they remain dry and have plenty of bedding material to snuggle into.
Dogs are a major cause of early death for pet guinea pigs kept outside.
In New Zealand the usual bedding is several thick layers of newspaper topped off with a thick layer of hay or straw. Shredded paper can be used but need to be changed daily as it becomes dirty very quickly. If bedding other than hay is used remember it will still be necessary to FEED hay to your guinea pigs. Guinea pigs are sociable creatures, and are usually all the happier for company, lthough they may ignore their humans more as a result. If you don't have a lot of time to spend with your guinea pig, or are gone for much of the day, your guinea pig may be a lot happier if you get him or her a friend.