what size top tube is on your single road bike? the rear size doesn't matter too much since the top tube length is usually the same no matter the seat tube size is and you wife is tall enough to fit most sizes. choose what's closest your road bike. i'm 5'11" and my wife is 5'7" or 5'8" and i bought a small burley with a 56 cm top tube since my road bike has a 55.5 cm top tube. the tandem has a 110mm stem while the the road bike 120mm to put me in roughly same position.
i just looked at the cannondale website. its kinda tough, one seems too small the l/s with a 56 cm top tube and the x/s or x/m with a 60.4 cm top tube. that's a big jump between sizes. if the larger one fits the medium rear will probably fit your wife better as it has a longer top than the small rear
http://www.cannondale.com/bikes/07/c...dem/index.html