Peter Jackson has stated that Smaug only makes a cameo in this first Hobbit film, so we'll have to wait until The Hobbit: The Desolation of Smaug to see the dragon in full and hear Benedict Cumberbatch's voice. As for Smaug's design, Jackson said the following to the LA Times/Hero Complex:
The trouble with redesigning dragons, I've found, is if that if you really get fruity, it starts to look like some kind of monster from another planet. You very quickly can go into science fiction territory, and I don't want to do that... I want to present the most venal, scary, decrepit, nasty dragon I possibly can.