Before revision: A larger or smaller idler wheel maintains the same surface speed (which equals the surface speed of the input shaft), therefore the output shaft is driven at a constant speed regardless of the size of the idler wheel (unless of course there is slippage, which should nor occur in most friction drive systems when operating correctly; however, there are instances where an idler wheel can double as a clutch, or if there is a sudden or unusually heavy load on the system.
After revision: A larger or smaller idler wheel maintains the same surface speed (which equals the surface speed of the input shaft), therefore the output shaft is driven at a constant speed regardless of the size of the idler wheel (unless of course there is slippage, which should not occur in most friction drive systems when operating correctly; however, there are instances where an idler wheel can double as a clutch, or if there is a sudden or unusually heavy load on the system.