The standard size of a conveyor roller can vary widely depending on the specific application and industry requirements. However, common diameters for conveyor rollers typically range from about 1.9 inches to 3.5 inches. The length of the rollers can also vary significantly but is usually tailored to fit the width of the conveyor system where the roller is to be installed. For instance, roller lengths may extend from several inches to over a foot to match the conveyor's width which could be anywhere from 12 inches to over 36 inches. It's essential for users to refer to specific product specifications or consult with manufacturers to determine the most appropriate size of rollers for their particular conveyor system, as the "standard" size can differ based on factors like load capacity, type of material being handled, and the environment of operation.