Have your wife try bikes at the LBS that are similar in build to the one you're looking at online. Better to be sure before you buy, especially if you're buying online.

I'm 5'5" with a 30.5" standover and I just bought a 17.5" Trek Allant WSD. This was after testing both the 15" and 17.5" models. My Tricross is a 49cm frame, which might be considered small for my height. You really can't tell without actually getting on the bike, so get as close to that scenario as you can by testing those that are similar.