That sounds good to me. Kirby drew him about 6 feet tall, so at 6'8" he'd be about 10% taller and 10% wider than Kirby drew him. Not crazy but enough to make sure he'd be an imposing figure.
The first time we ever saw the thing, he was smashing through a door that he was too wide to fit through, and that was pretty cool. But for at least the next 10 issues or more, Kirby drew him at a size that wouldn't have any trouble getting through a door. I'd love to see this film feature some variation of that door smashing panel. (if I remember correctly the dialogue was something like 'B'ah, why must I live in a world built too small for me.'
I don't want him to be 8 feet tall or anything crazy like that (and some artists have drawn him close to that), because then he wouldn't be able to fit in a normal room or function relatively normally. But at 6'8" and in proportion to the way Kirby and most other artists have drawn him, he'd be very big and imposing, but able to function.
. . . and consistent with how he has been portrayed by most artists since Kirby.