The wheel should be large enough so that your hamster does not need to arch its back when using the wheel. A Syrian hamster will therefore need a wheel that is at least eight inches, while some will need a wheel as big as 12 inches when fully grown. Most smaller wheels should be suitable enough for dwarf and Chinese hamsters, but it is recommended that you buy a larger wheel if possible to avoid any problems, as these are still easy to use for smaller hamsters.
You should provide your hamster with a wheel that has a solid running surface and a solid back. A runged running surface can be dangerous as if your hamster slips it may catch a foot between the rungs while the wheel is spinning, causing injury. Many cheaper wheels also have supporting sidebars across the diameter, which can again cause injury if your hamster sticks its head out when using the wheel.
Since your hamster is likely to use its wheel a lot during the night. You may want to consider the noise of the wheel, especially if the cage is kept in a bedroom. A small amount of vegetable oil can be safely and effectively used to grease the wheel if it squeaks.