Height is pretty irrelevant on screen really; just look at Peter Cushing as Sherlock Holmes, Humphrey Bogart as Philip Marlowe or Daniel Craig as Bond for that matter. Also neither Roger Moore or Ian Ogilvey are that tall.
Although I appreciate the camera can get away with a lot, if the actor is the right height then it's one less thing to worry about. It's evident how important height is to Bond when one realizes the first five actor were all over the benchmark of 6'0.
I'm not sure about the heights of some Saint actors, but IMDB records the following:
Louis Hayward as 5'10½ (5'9 according to Burl Barer, the Saint biographer)
George Sanders as 6'3½;
Hugh Sinclair ? But he looked at least 6'0
Roger Moore 6'2 (6'1 elsewhere)
Ian Ogilvy 6'0
Andrew Clarke ? But I think he's definitely over 6'0
Simon Dutton ? I'd guess around 5'11 to 6'0. Ian Dickerson might have a better idea, as I think he's met him.
Val Kilmer 6'0
Sorry, quite anal of me to start listing heights, but I am curious. They're mostly on the tall side, though.
For what it's worth I don't think it's Sheen anyway!
As for Branagh; he's pretty much tied up with Wallander on tv and the upcoming Thor feature film (plus is now in his 50's).
I don't think it's Sheen either, as his career is doing too good at the moment on the big screen. I don't think Branagh would have much of a draw these days.