As the other posters have eluded to, you are on pretty sketchy ground and sound like you have a good chance to wind up with a 40lb garage decoration like 90% of tandem purchasers (who are mostly cyclists, who want to get thier spouses more active). My parents fit this description to the t, My dad was so into it and my mom thought it would be neat (she is not active, he was a recreational cyclist). Of course I think the longest ride they ever did together was like 4 miles, probably less than 10 times in 5 years. I did ride it with him several times (a tandem is fast as hell down hill with two 6'4" 200lb+ guys on it), and I ended up putting a couple thousand miles on it with girlfreind when I lived in the area(so I was either the stoker or the captain for 99% of its use, while my mom was only about 1%).
I bought one with my current girlfreind and it is OK, but she was a cyclist before we started dating. Even though she is a cyclist too, you still have to get used to the control/communication issues. I have to force myself to not get irritated by her lack of technical knowledge of a bicycle for example when she says things like "the speed isn't right, do I need to lower or raise my seat" or similar comments that make no sense to me. But while she is strong and fast for a female rider, she can only follow me on singles on my recovery or easy days so the tandem is a great equalizer. It is a blast to go on group rides and dust singles plus the positive attention you get from motorists, etc.
I would be VERY cautious before dropping some money on a tandem if your wife doesn't even ride. For sure rent or borrow one and ride it several times for long rides, and don't push her at all to go. Even if she likes it while you borrowed it, you may find that you are the only one motivated to ride and she is just placating you, which will end you up in the situation many tandem buyers find themselves in. If she really really really likes it, then it may be a good purchase. If not, consider just going for walks together or something else you can enjoy together.
Regarding the size difference, it isn't that much. I know Cannondale makes a couple of sizes that are very different and just off the top of my head the frame in (L/S) should fit both of you with some minor changes in stems, etc. I'm sure other makers do as well. People get too anal about frame sizes anyway. My tandem is a 58cm for the captain and I am 6'4", but with a mountain seatpost and 150mm stem I have no problem even though it is about 4cm smaller than I should be on. If you both compromise fit slightly (i.e. you get long stem, she deals with minimum standing clearance) you should have no problem finding a decent rig.
Good luck.
