I find it easier to imagine him as Jamie. But actually I think King often doesn't describe what his characters look like that precisely. He describes their clothing and their demeanor, and what kind of impression they make on people, but not so much else. He gives details sporadically, he doesn't go by every feature like hair, eyes, nose, mouth, length, if they're thin or heavy etc. He will mention details mostly when there is something special about it, like after Jacobs had a stroke, one half of his mouth wouldn't work anymore.
Some writers completely describe every feature when a character is first introduced. But I think mostly he doesn't do that, so you imagine them more yourself.
I would say, watch the first adaptation. It was written by SK himself and filmed according to his wishes. Although there are clear differences with the book, just don't expect to get exactly the book. Although: would people be satisfied if something is exactly the book? Because then the film wouldn't hold any surprises or anything new. I thought the first film captured the atmosphere of the book at least, and based on the trailer this does too and seems to expand more on the book.