Sheldon,
It's so nice of you to get a jersey for your wife! She weighs less than I do, so I think you need to go with the smaller sizes. A few brands run big/small. It also depends on whether she like a club fit or a race fit.
To give you an example, in tops I wear a small in Castelli and Assos.
I wear an X-Small in Giordana, Shebeast, Primal, Gore, Craft, Descente, Sugoi, Louis Garneau.