David Prowse (Vader) was 6' 5-6" in his prime. The costume was worn at a height of 6' 8" for a man with a 260lb bodybuilding frame. They do have different size parts out there, example, chest armor. The guy had a 53" chest, so 1:1 scale armor can be tricky, there is a smaller size set out there. However foam padding can help too, or time at the gym.

The only size that can't be played with is the helmet. Good thing to bear in mind, this costume is a heavy one! The cape can weigh up to 10lbs, the helmet if fiberglass can be 9-13lbs, etc,.. I know my whole Vader weighs in the ball park of 37-43lbs, and a strong back helps!
This being said there are many Vaders of different heights out there in the Legion.
He's a very fun character, but with the glory goes the suffering.
