Surly puts 26"(559bcd)on it's smaller frames because it is difficult to design a small frame that will accomadate 700C wheels with wide tires and fenders . Toe overlap on a touring bike can be a problem.

Another possibility would be 650B(584bcd) wheels. The SOMA Grand Randonneur is one example. Rivendell also has a couple of frames designed around 650B wheels. They are more expensive, but will probably last decades. There are others such as Velo Routier from Canada.

There will be posts after this saying "But the availability of 650B tires is limited". That was true a few years ago, but its less true now, with the popularity of 27.5 .

Disc brakes? Dunno. Velo Orange is coming out with a new Polyvalent designed for discs and 650B.