i am 6'1 and my wife is 5'0. found a used tandem size is.
captain- 20 inch frame 30.5 stand over height 21.5 top tube.
stoker- 19 frame 29.5 stand over height 22.5 top tube.
thanks for your help.

I think that you are going to be very cramped on it; in height as well as length. It might be a little large for your wife, but there are alot of things that you can do to accommidate her, such as a longer reach stem, but her seat height might not leave room for a shockpost, if that is something that you might need. It will probably work better for her than for you. You really need to do a test ride.

Sounds kind of big with 19" in back, but you can't strictly go by your height. The best way to tell though is go give it a test ride. If it fits, decide if you want it. If not, let it go. The link is completely ball park and by no means necessarily correct. You can possibly get away with a little big for the stoker as they don't have to put a foot down when you stop, but they do need to be comfortable. FWIW, I am 6'2" and my fiancee is 5'4". Our tandem is a 20" in front and a 17" in the rear and fits us perfectly with a decent length stem and a couple of inches of saddle to bar drop. I don't feel cramped and she doesn't feel too stretched.

https://bicycling.about.com/od/howtor...MTB_sizing.htm

Yes, I think the rear is definitely too big for her. I would think she needs about a 16". Be patient, the right one will turn up.

If it's local and you can try it out, then I'd take that opportunity, maybe just to learn what that size is like. But I don't think you should buy it. FWIW, my wife and I rented a tandem bike about 3 years ago. She loved it, but we just enjoyed our 1/2 bikes for about two years until the right tandem showed up that we couldn't refuse. Take your time shopping and do some test rides.

Probably a bit small for you and a bit large for her, especially if it's a male/male configuration.
A test ride would be in order . . .