Try to go for characters that are physically imposing, rather than the actual height of the actors.
So, any Ahnold character would work (if you're buff enough).
Also all of the characters mentioned so far (Vader, Wookiee, Jason, etc.).
If you want to do a period outfit, a member of the Grenadier company of any 18th or 19th century British regiment.
I'd figure Robocop would work too. Yeah, Peter Weller was a 5'9 skinny dude, but just think how impressive a 6'6 225# Robocop would be!
Oh, Predator, too. Definitely tall enough for that.
Along the same lines, US Colonial Marine.
Alternatively, play with the height thing and do a 6'6 version of anything small (As mentioned -- the Umpa Loompa). Also an ewok/jawa/member of the lollypop guild with a glandular condition.
You could probably pull of a William Wallace, too.
Come to think of it, you could DEFINITELY do an Edward I. He was about 6'6 actually, if memory serves.