from the pics we have already seen, i have noticed some size discrepancies. Vader's "head" now looks a lot larger compared to the rest of him than it did in the OT. I wish that Hayden had not foiught so hard to get to wear it in the new film. Or maybe i wish they had at the very least, forced him to stand on a boc.
Luckily, Vader's arms and legs are all artificial. This is my reasoning for telling myself that in the 19 years between Episodes III and IV,, Vader decided he wanted longer arms and legs

(More reach while fighting? It appears that Grievious' cyborg/robot body is larger than his original one)
In fact, in my head i would love to imagine that Vader even had a Grievious style 4 armed body for a period of time yet to be documented. Ahhhh the joy of imagining a 4 armed Vader slaying Jedis in their hiding holes!