The most important measurement for picking a condom is girth, not length. To get a very rough estimate, slide a toilet paper roll around your erect penis. If there is extra room, you qualify as "small." If it fits about right, then "medium." If it is too tight, "large." Cool trick. That, or use a tape measure or a piece of string to measure from the base to the tip of the penis while erect. Take the width measurement as well, around the penis at mid-shaft and use that measurement to determine your girth. Remember, the more snug the condom, the more smooth the ride.