Most hutches have a frame constructed of wood, including legs to keep the unit off the ground. The floor may be wood, wire mesh, or some combination of the two. One or more walls of the hutch are also wire mesh to allow for ventilation. Some hutches have built-in nest boxes and shingled roofs — these are generally intended to be placed directly outside rather than inside another shelter such as a barn. Some hutches have a felt roof. In any case it is important that the hutch is draft-free and provides a shelter in case the animal is scared and wants to retreat to a save haven.
In terms of sizing, guinea pigs should have at least a 4x2ft hutch, so that they are able to move around conformably and lie in any direction. They should have plenty of time out of their hutch every day. However, rabbits need at least 6x2x2 ft hutch space to enable them to take at least 3 hops, stand upright and lie in any direction. In addition, they need at least a 6x4x2ft run attached to this all of the time, so that they are able to exercise at any point of the day. More information here: http://boardingbunniesathome.co.uk/rabbit-accommodation-ideas/
- BSAVA manual of exotic pets, 4th edition