Guinea Pigs like to have a defined range they can call their territory. "Wild" Guinea Pigs live in tall grassy areas and have a range of roughly one acre. Since they are relatively defenseless, Guinea Pigs develop a trail system in the tall grass so they can out run, out maneuver and hide from enemies. This is why Guinea Pigs are so sleek and explains why they make "bocking" sounds and walk around in single file. You can also observe the trail instinct, especially in pigs under one year old, when they run a regular pattern over and over again as if they were running through the grass on a trail. If you have a new family of pigs you can see this trail behavior at about one week. It is very exciting and funny to watch several piglets run "trails" at the same time.
Guinea Pigs like to run and explore so it is best to have as large a home (or pen) as possible. Just think of how unhappy and unhealthy you would be if you were only allowed to stay in one small room all day.
We had started with one pig, Emma, in a cardboard box, lined with clean newspapers and a pine chip bedding. We would get clean boxes from our grocery store and tape several small boxes together to make one large box with tunnels and gateways for her to play in and climb over. Guinea Pigs love to climb over 4" walls and walk through doors that they can barely fit through. Oatmeal cartons make a great tunnel and hiding place. See if you can think of special games or mazes for your pig to try and play. Remember that pigs usually do not like to walk on smooth surfaces because they get little traction. They also really like to have a place that they can get their head covered.
The size of the boxes was roughly 36" x 28" and we cut the top to 10" high in the summer to 6" in the winter. Drape a towel over one half of the box. Guinea Pigs get very nervous if they do not have a place to hide their heads. The towel is also a nice cover to keep the animal warm at night. Change the towel every 2 to 3 days to keep it clean. White towels are best because you can wash them in hot water soap and Borax to get them clean.