I like this pic cause it clearly shows leia, Carrie Fisher 5'1" only comes up to the bottom of Vader's neck on his mask.
This pic shows she only come up to the bottom of Hans, Ford 6'1", chin.
Now I don't have any pics of Luke next to Vader, Han, or Leia but I do know that Hamill is 5'7" and can just see over Leias head which makes sense if he's only 6 inches taller than her. Which puts him right about at the bottom of Vader's chin.
Han can see over Lukes head in several scenes, from what I've gathered, so it would make sense that Han would come up to maybe the middle or top of Vader's eyes.
Which would also make sense if we take Peter Cushing's height into consideration which is stated as being 5'11" or 6'. And as we can see referring to the first pic Peter Cushing looks like he only comes up to possibly the bottom of Vaders eyes.
This pic shows Sir Alec Guinness, 5'10", just about right at 3 inches taller than Hamill. He can't quite see over top of Luke's head which would make sense if Ben is 3 inches shorter than Ford.