Wheels are a great form of exercise and enrichment when the correct size!
Why is it important to properly size the wheel to your rodents? 1) A wheel that is too small will cause discomfort and can lead to spine injuries/wheel tail. 2) A wheel that is improperly shaped will cause discomfort and spine injuries(saucers!). 3) There is no such thing as having a wheel that is too big!
How to properly size a wheel to your rodents: 1) If there is more than one rodent in the cage, choose the largest one and measure them from nose to tail tip(doesn't have to be exact, but as close as possible. Its always best to round up, not down!). 2) If your rodent is 17" long, you will need a 17" wheel. If your rodent is 10" long you will need a 10" wheel.
Pretty straight forward! If your rats are still growing, its best to just get the biggest size you can. Females usually get to around 14-15" long and males usually get to 17-18" long. As they grow just keep track and upgrade if/when necessary.
How to choose what brand and type of wheel! - You want a wheel with grips on it. You don't want your rats running full speed on a slippery surface! - You want a wheel that has a full running plate. You do not want a holy or grated running surface! - You want a wheel that is open! Wheels with front plates with access holes are dangerous!
What types of wheels to avoid! - Mesh wheels! These cause foot injuries and infections! - Saucer wheels! These cause spinal injuries!