As you might already know, the F650GS, F700 and F800 are all 800cc engine, just different tuning for horsepower output.
I have a 2011 F650GS and would not hesitate to take long trips 2-up on it. Unfortunately my wife is no longer interested in riding on the back of a motorcycle.
I cannot understand the thinking of someone who says you *need* a 1200 for 2-up touring. When my wife and I started touring in the '70s the biggest bike we owned was a Suzuki GT750. It had much less horsepower and much more weight than the F650/700/800. But it took us and our gear on extended touring trips with no problem.
My F650GS has a far better power-to-weight situation and is very happy the occasional time my wife agrees to come for a ride.
I have the factory-lowered model with low seat. I do not recommend this model for your situation. The rear suspension is set up for very light weight. I am 5'6" so you are a little bit taller than me. The standard model should fit. If you feel it is a little bit too tall, just buy a lowered seat.
I have tried my lowered seat on my friend's standard model and I could easily ride like that.
Even riding solo I have ordered a new rear suspension with a stronger spring and a little bit longer shock to give the bike some more ground clearance. If I was doing it again I would buy the standard bike with the low seat and be very happy. I'm 185 lb. with all my riding gear on.