Dave Prowse was (depending on which film you are going for - and indeed
if you are going for Prowse size) 6ft 5in tall during the shooting of the
OT, not to mention his build and stance.
I seem to remember a heated debate about Prowse's size during the OT, either here or over at The Porp Den a while back :rolleyes
But during an interview with Prowse himself on a documentary made in 1980 here in the UK (Clapperboard) he stated that he was 6'6", so I reckon he was this height during the whole O.T. since he was a fully grown man back in 1976, and his height wouldn't have increased or decreased in 6 years! :lol
Not that an extra inch in the height of your mannequin would make *that* much difference, but it might have an overall effect on the look of the display (also according to how high the helmet is sitting, armour placement etc.)
At least with a Stormtrooper manequin the proportions are approximately right to begin with, but modifying a mannequin for a Vader display would be a pretty big job:unsure
If your stock mannequin is 5'10" then remember to spread the extra 8" over the legs, abdominals, chest etc., otherwise you'll end up with an out-of-proportion display.
I've no idea on what length to extend the arms other than extending the main mannequin first, and then seeing how far down the elbows / hands hang in relation to the belt etc. - then guess how much to extend based on that.
Provided you're not planning on re-posing the arms every other day, then I would recommend coathanger wire fibreglassed into the inside of the hollow elbows, where you'll be cutting them. That's what I did with my customised trooper mannequin.
Keep us posted on your progress, and if you need any help, ask GINO - his display looks pretty damned good, and in proportion :thumbsup
Cheers,
John