Nice looking 21 speed Cannondale road tandem for sale in Everett, WA. Only $1200 Got this link of Seattle craigslist. They show a pic but don't list the size. Happy Hunting!
http://seattle.craigslist.org/bik/121126880.html

I asked what size it was and this was the seller response:

"The distance from the center of the crank to the top of the seat post is 21 inches for the front and 19 inches for the stroker. I failed to mention that the shifters are located on the handlebar tips."

I believe that is a M/S in Cannondale's sizing scheme.