As alluded to in my earlier faux pax on frame sizing, Cannondale changed the sizing on its tandems back in 2009.

I had to go and look them up to be sure and still haven't found the old sizing tables, but...

New sizes are: Jumbo/Medium, XLarge/Small, Large/Small and Medium/Small

Pre-1999 - 2008 sizes were: Jumbo/Large, XLarge/Medium, XLarge/Small, Large/Small & Medium/Small.

Note: That's also why the Small (27.1") stoker compartments were on my mind, as they did seem to invade more than 1/2 of the frame sizes.