M-PETS Training Wheels. In the wild, small animals can run long distances as they have a natural instinct to need to exercise daily. A wheel is the ideal addition to an enclosure to provide a safe place to run. This wire wheel can stand alone or be clipped onto cage bars. Sizes: 18cm(7") Diameter : recommended size for smaller syrian hamsters, gerbils, mice 27cm (10.6") Diameter : recommended size for medium syrian hamsters, gerbils, mice.