You can go a little taller to 22 or down a bit to 18 but the 8 is rim size so you can not change that. Changing tire size also changes how the mower will react. Smaller tire size will make spinning the tires a lot easier. Going larger will increase top speed but make it easier to break transaxle. You did not include the tire width which can some times be wider. You have to make sure you have clearance for it as well. For every inch of taller or wider you need 1/2" of clearance. Just think it through before you buy. If you find something you want just come back here and ask again.