Lots and lots of good advice here...
My take - you will almost CERTAINLY progress to the 1200, and given your size, will be very comfortable on it...
The BEST money you can spend is on rider-training, and decent (not expensive) but decent safety gear, that will last...
The advice to start on an older 650 is sound advice, FAR cheaper to repair when (not if!) you drop it, etc...
An easier introduction into biking than straight into 1200 territory, IMO...
The much-underrated 650 TWIN and 700 TWIN often sell 2nd hand at a discount greater than that existing between them and the 800GS (when new) since they are often (not always) seen as ladies bikes, and many want the 800 rather...
Interestingly enough, the now-discontinued 650 twin out-performs the 800GS through the lower gears, when the 800 then outperforms it ( I MUST find this reference!)
And the 19" front wheel is more forgiving/turns in easier on road than the 21" 800GS wheel.
Flat-footing a 1200 on level ground is useful, yet we have a good few customers who drop newish 1200's outside our Store; the parking slopes SLIGHTLY down, rider tries to put left foot down, it's lower than he expects, bike starts moving, and BANG, bike goes down! Not once or twice, but with surprising regularity!
Old 650 single - if it's had a couple of owners, you will hardly lose money on it if you buy 'well', and get confidence in riding something more manageable.
The only caveat to this advice, is that you are likely to progress to a 1200, so as a last resort, you could start there, but I would start smaller/lighter/ cheaper, and invest in a course or two.
Enjoy, and be safe!
Chris