If you want to sell Thor to the movie going audience, I suggest they pick Brad Pitt (Troy) and leave the rest to CGI...
Stallone is 5'7 yet he was able to play Judge Dredd. There are several film technologies/techniques which can manipulate/enhance the size of the actor portraying a role/character bigger than his frame.
Brad Pitt would really sell this movie to the viewers unfamiliar with The Mighty Thor. As for us, the comic fans, we get the chance to enjoy the film played by one of the better actors in the movie industry today.
Just imagine Brad Pitt crying as Thor ala Troy when he is banished by Odin... great stuff!