13.25 inches looks best with the other short figures in HT's Star Wars line.
Look at Vader in the movie and then look at your Vader figure with your other figures. It's quite simple without math.
I never understand why people rush to google to check actor's heights (when they lie) instead of checking the source material (the movie) and eyeballing it.
But it's not that easy.
-It wasn't always Prowse in the suit. Bob Anderson did most of the lightsaber sequences, so it isn't easy to know when it's Prowse and when it's the stunt guy.
-Mark Hamill sometimes wore lifts on his boots, so it isn't easy to know when he is or isn't.
-Camera angles and other filmmaking tricks make it difficult to know if the characters are directly next to each other, etc.
For example, consider the Emperor's Throne room sequence.
First we have Luke and Vader in the elevator. We can assume they're standing next to each other, on the same surface. Here, Luke seems to come up to about the bottom of Vader's nose.
Jump forward to when they reach the top of the stairs, and we get this angle. Here, Luke seems to only reach the bottom of the breath screen:
A few seconds forward, and we get a different angle. Here, Luke seems to come up to the bridge of the nose, maybe the bottom of the eyes.
Cut back to the first angle, and Luke is again to the bottom of the breath screen:
Cut back to angle 2, and he's back up to the bridge of the nose:
Later, a wide shot. Again, maybe to the bottom of the breath screen:
Here's a behind the scenes photo from ESB, you can see the lifts on Mark's boots. But, neither of them are standing up straight, so it's hard to judge.
And, a behind the scenes photo of Mark and Prowse. You can see in this one that Mark comes up to Prowse's chin, so the shots of Mark/Luke coming up to Vader's nose have been meddled with somehow:
And, a shot from a deleted scene of Luke and Vader arriving on the Death Star. Here, it looks like Luke comes up to his chin, but perspective and parallax error could throw that off:
So, camera angles, forced perspective, and other tricks (standing on boxes, lifts, etc) can throw it all off.