Well, the best way is to take one for a test ride to see how it "fits" for real. However, if that's not an option, on the M/S (19"/17") road tandem the top tube is 54.6cm which is in the range of what you'd typically find on your average, stock 54cm single road bike (CTC) -- so it looks like it is probably the right size. The next size up, L/S (20"/17") has a 56.1cm top tube which is what you'd find on your typical stock 56cm single road bike -- probably a bit too big even though the seat tube length and standover are OK.
As background, the standover height and top tube length are the ones to pay the most attention to when shopping for tandems, except on the Cannondales. Because of Cannondale's funky design, the seat tube length and standover heights give the impression that these bikes will fit shorter captains than they really should (or that you might need the next size up). For example, the M/S has a standover of 28.3" which is what you'd find on a 48cm single road bike which would likely have a top tube of about 51cm.
To take a look at the rest of the dimensions for Cannondale's tandems, visit their Web site:
Road Tandem: http://www.cannondale.com/bikes/02/geo-13.html
Off-Road Tandem: http://www.cannondale.com/bikes/02/geo-12.html