Lots of big guys here.
I am 6'7". David recommended 66" in grenadine grossa. I insisted on 68" wanting to make sure it was long enough. It is borderline too long. 66" would have been better. But in the future I will stick with 68" for a non-grenadine tie where I want the option of four in hand or half-windsor. David also recommended 4" width and that is perfect.
Moral of the story is to ask David and trust his recommendations.