Join Date: Jul 2006
Location: Buenos Aires, Argentina
Bikes: Co-Motion Mocha, Trek T100, Schwinn Fastback Comp, Specialized Stumpjumper
You should not have any problem unless you run on a very tight budget. Older tandems and cheap tandems typically have a small stoker compartment, so you start with a small bike fitting your captain, and you end up with an even smaller stoker compartment which is your concern.
However modern tandem manufacturers will have sizes to fit you, and there are some tandems which are specifically designed to take a broad range of riders in either seat, check out:
Either of these bikes should fit both of you and you will find teams in this forum that ride them.
Is your wife a cyclist? Many of us are in the situation where the captain is a cyclist and we drag our dear beloved onto the sport, as a stoker, with varying degrees of success. Most of us on this forum are captains. Teams that give up are often due to the stoker quitting. This is certainly a generalization with many honorable stoker exceptions who will take issue with me. You have a big advantage over the rest of us if you as a stoker take the lead in this project. Best of luck and hope to hear more from you.