I'm 5'8" and solidly a medium on most brands. I avoid Pivot myself because their medium is usually huge, and their small small, so maybe their small would be goldilocks size for you. I personally really like the Santa Cruzes I've owned, but one issue I'm having with them is their shock tunnel prevents me from running a 200mm in a medium, fully slammed. You might be able to get away with a 180 on a small, not sure. I looked at the new Yeti LT but while they figured out the straight/deep seat tube, they made it 420mm long...