What are the standover heights that you need? A medium with 26" wheels might work for you. I'm 5'8" with a 29-30" inseam and can stand over both our Co-Motion Mocha and Santana Fusion 26" wheeled tandems; my wife/stoker is 5'2" and around a 28" inseam and the mediums are ok for her, too. Standover height in the back isn't quite so critical for most stokers.
Small-sized tandems are hard to come by used. I'm keeping my eyes open for a good one with couplers as I have some back issues currently that make the medium sizes just a bit too hard to throw a leg over and the small might work better for me in the near term.