All the major tandem companies also make custom sized frames and have a wealth of experience. I think that matching the geometry of a single bike gives different handling on a tandem, so no one can extrapolate what they know about singles over directly. Talk to some tandem riders about the differences in how those major brands handle and then pick your company.
I think this gives you the best bet for getting the bike you want, and at the best price.
I saw a rolling resistance test comparison between a bunch of tires and what I saw was that the differences due to size were swamped by the differences between models. I suspect the same is true for rim diameter. The overall diameter difference is not huge and rolling resistance is not directly related to diameter.
But for wheel size you can really just go with your personal prejudices. I think you can see that whatever you chose there will people here who agree 100% and people who think you are dead wrong.