It was clear to me when shopping that although everyone voiced the usual concern about "fit", that when it comes to the frame, it's really about getting the bike fit right as normal in the front compartment, and the only really important statistic on the rear compartment was standover. There are a limited number of boom lengths, all longer than needed, and the slight differences are between companies, not between sizes. (That's why Gates is able to offer one tandem timing belt with distance to be adjusted by choice of ring size.) Tandems East or other shops will gladly sell you a nigh-infinite variety of stoker stems and seat posts, to get the reach and stack to be whatever she wants.