Good work (getting a positive from t'other half). Are you talking retro or modern?
Modern is easy, either xs or s - all depends on leg length / arm length etc and the preferred position. The modern steeds obviously tend to have the more relaxed or sportive orientated geometry so offer a more cosseted riding position. Anything with a horizontal top-tube, i.e. retro will be more racey to ride and perhaps not as comfortable to a novice. You could be anywhere between a 48cm & 51cm dependent on the specific geometry etc.
Hope this helps.
