They run how they run. Honestly, if you want to buy your wife a jersey, you'll have to figure out what size she is first.
If she's not "Gargantua, frankenstein of the north", don't get a large. It will be way too big. As far as a medium or a small, who knows? Only you know her size, and on top of that, with jerseys, some women prefer them loose and some prefer them tight. You have to know that too so you don't end up getting her something that fits funky for her taste.
Good luck...
Koffee