100% approved by our army of gals and our field report earns us a few bucks.
Get her a frame. To a gal, a bare poster is an unfinished gift. Put it in a nice frame and you've got a winner.
Pick an appropriate size. Try thinking ahead about where she might choose to hang it up. If there's a good little spot in the kitchen, then get her a smaller sized poster. If there's an entire blank wall where it might go, then get her a larger size.
Don't hang it up ahead of time. If you try to surprise her by hanging it up first, and then revealing it to her, that's not a gift...it's just another home decoration. You need to wrap it up and hand it to her.
Don't suggest where to hang it. If she picks out some oddball place to hang it up, don't say a word. You may not like her choice, but it's not your gift. Give her complete control over where it goes.