Well, anyone reading this thread and wants to try Reacher, you must read them in order, otherwise you'll miss all the character development and a lot of things won't make any sense. The current book is #17.
Travis McGee had this trouble also, a couple of weak attempts to put a GREAT series onto the big screen, really poor selection of actors. I always thought Ron Ely would have made a great Trav.
Casting of characters is terrible. Some authors won't sell movie rights unless they have control over casting, which is hard to get.
My pet peeve in this regard is, "Live and Let Die", loosely based on Fleming's terrific book of that name. Ian Fleming descibed CIA agent Felix Leiter as " a straw-haired Texan." That film turned him into a black man!

This PC stuff is just totally out of hand. Fortunately, Leiter is better cast in the other Bond movies, but never as he should look or behave. And some of the actors playing Bond were not up to the role, either.
You know, Tom Cruise's ex, Katie, would be a good Bond girl. Sharp and pretty too, and a very good actress.

If she ever gets that role, remember that you first read it here, and that it was my idea.
I've never read a Jack Reacher book. All I know of him is what I read here. But he seems a sort of
macho chest-thumper designed to appeal to rednecks insecure about their virility.
As for his size, read Jack Higgins's books featuring the former IRA assassin Sean Dillon. And Gen. Chas. Ferguson. Ferguson runs other agents who also kill ruthlessly and effectively, although most seem to be of average to a little taller size, and pretty polished. I think Dillon is about 5' 6". But he is a very tough guy, whose exploits are quite plausible. Keep in mind that Col. John S. Mosby, CSA, was not a large man, either. But he had even Lincoln worried that he might come calling, Colt .44 in hand.
(For the record, I am not a small man, but not especially big, either.)
As for lethality, the Modesty Blaise books by Peter O'Donnell make it clear that size doesn't have to be a requirement for being very dangerous. To be sure, Modesty's male pal Willie Garvin was a big Cockney guy, but the author didn't get off on his physique. I must say, Sean Connery was well cast as Bond, though.
David Lindsey's Houston homicide detective Stuart Haydon was about six feet, but his image wasn't redneck macho. Far from it: he was very cultured and his wife was a talented architect. He wore designer suits and carried a Beretta. He had family money, as a normal cop couldn't afford his home, car (a Jaguar) and other trappings. But Lindsey made him seem very real.
George Peppard could have probably played this Reacher guy, but he's long dead. Most modern actors are pretty bland. My daughter thinks some on the fantasy shows with demons and vampires are pretty hot, though. I think she likes Russell Crowe, too. And Mel Gibson. Harrison Ford is too old now for a lead male character. But Liam Neeson is hanging in. He has a new sequel to, "Taken" due out soon, if it isn't already in theaters.
Oh: Travis McGee was mentioned. I never read those books, as the author was anti-gun. But didn't Rod Taylor play McGee in at least one film? I'd have thought he would be pretty good back then.