Drive sprockets, rather than being measured by their circumference, are typically measured by the number of sprocket teeth they have for a standard size chain. Clutch sprockets tend to be slightly smaller than final drive sprockets, sometimes a fifth the size or smaller. You may find this counterintuitive, but to go faster, the final drive sprocket needs to be smaller, not bigger. On the contrary, to go faster, the clutch sprocket must be larger. So a simple way to change your gear is to have one or more primary drive sprockets and / or additional final drive sprockets. Remove the chain and replace one of the sprockets with a sprocket that will increase or decrease the speed of the wheel.