Not a sizing issue, but something to consider...If your stoker is a former rider, and especially if he is/was into data like "how fast are we going" and the like, consider a Sport-IIII (pronounced "4 eyes") and a speed/cadence sensor like the Garmin GSC-10. The Garmin unit will send speed and cadence information wirelessly (via ANT+ signals) to bike computers and other devices designed to connect to ANT+ sensors. The 4-IIII is the beauty of the system, it is a small device that attaches to the arm of your sunglasses. It has a number of LEDs on a small flexible boom designed to be seen in your periphial vision. For the blind (although it was not initially intended so), it has an audio output that can be programmed to give speed, cadence, and power and heart rate if you have those sensors, either when the device is tapped with a finger or in preset intervals. Keeps me from constantly having to ask my pilots how fast we're going all the time. These units are now used by the British Paralympic cycling team (at least the blind tandem teams), and their website is
www.4iiii.com
Not every stoker really cares about the numbers... Sure helps me pass the time, and regardless of the team's riding goal, I can dose my workouts (heart rate zones) and get in some productive workouts.