tjf9, my Dh borrowed an Xtracycle this past spring from one of his buddies that used a step-through mountain bike frame with 24" wheels (I think, or maybe they were 20" ). At 5'8" he could ride it pretty comfortably and the frame was small enough that I could probably ride it too with the seat lowered and the handle bars angled toward the seat a bit. BTW I'm 5'2" with a ~27" inseam and I could straddle the top tube comfortably. Here's a pic for you: