I am not tall, 5' 11" and about 31" inseam. When I was out shopping for a ST, I sat on the FJR and a ST1300 - did not take test ride though. I was able to pretty much flat foot both bikes. The C14, I could not flat foot. Judging by that, a tall person would be more comfortable on C14. At that time, the C14 was more appealing to me due to being a more modern machine than the ST1300 or the FJR (this is 3 years ago). So I test drove and bought that thing (even though I was tippie toeing). Since then I lowered it just a bit to make me comfortable.
Recently I test rode a K1600GT as I was contemplating going down to one bike. Don't know if I just got used to ST riding, but it did not fell as tall or intimidating as when I first sat on the C14. I would venture to say they are of similar height, just that on the K1600GT you sit more upright on stock setup. The ride was real good - bike felt more nimble. Power delivery was smooth, but did not feel as instantaneous as the C14 (even in dynamic mode). Gearbox clunks like my harley davidson (almost) - not too big deal for me, but was odd for such a high priced bike. At the end, both wife and I decided, not worth it for twice the money as my C14. She had a renewed confidence on the C14 because she could not believe how well the C14 rides compared to the K1600GT for half the price. I know this is K1600GT, but I would imagine ergonomic wise a K1600GT or a R1200RT will not be too far off.
So my personal opinion, I do not think you will have any issue with C14 as far as your height is concerned. My brother-in-law (he is 6'4" and about 260 - 280 lbs) rides a HD Road Glide, which is like 200 lbs heavier, but when you look at his bike and the C14, the C14 sorta dwarfs it - honestly.