For two up travel I would say nothing less than 600-650. Closer to 1000 would make the tarmac stretches that bit easier to negotiate: better cruise speeds, and more torque for lively overtaking.
There are plenty of models from the Japanese "four", BMW, KTM, Aprilia and Guzzi also have dual purpose bikes of GS ilk...
We toured around Argentian and Chile, two-up on an R1150GS. It handled everything very satisfactorily, indeed. Off-road was not a breeze, but then it was a heavy bike, with two people and I was new to dirt. So it may not have been pretty, but it did the job, and of the two, I was the limiting factor not the bike!
Note, however, that I have seen Yamaha Diversion 900s doing dirt, as well as a H-D Road-king electra-something, so it's all possible.
I would firstly say, don't restrict yourself to current bikes: buy second hand: you'll save a packet that you can spend on getting stuff, or getting to places!
Also, don't limit your choices on paper, either. Get out there and test ride as many new, or used bikes as you can. See what feels good, what gets your juices flowing etc, so don't just think specs.
As for the 1200: many people have embarked on long journeys with these, but it is a lot of cash which is a lot of tankfuls and hotels stays. Personally, I bought new a long time ago: never again! If you do go that way, look for used models: they may already have half the bits yiou want bolted on for you!!
Now go test-ride anything and everytthing that looks remotely like an option: you may be surprised by what revs your engine!!
Adventure: it's an experience, not a style!
(so ride what you like, but ride it somewhere new!)