Camera angle and wardrobe can make a VERY big difference in how an actor appears in a film. Just look a Bogart. He was a tiny guy. Wiki says he was 5'8" but I've seen several bios that list him as 5'6". But in every movie he played, he came off as a much larger person. They say that in one scene with (can't remember if it was Bacall or Hepburn, he had to stand on a box so he could be shot looking down into her eyes...
I think they will be able to make Tom Cruise work, though I'd really have preffered to see a young Arnie-type.