Danny Huston played him in Orson's Shadow, right? I saw it here in Chicago, and while he did Orson's mannerisms & voice very well, to me he looked more like Raymond Burr.
Vincent D'Onofrio, definitely; I plowed through Ed Wood just to see that scene, and was almost convinced they'd done some complicted CGI, he was so good.
Robbie Coltrane....hmmm....I suppose so, physically at least. I love Robbie, but somehow just can't see it.
Okay, here's one out of left field: Chazz Palmintieri. He has the height and the acting skill---and he (and Vincent D'Onofrio) have the added advantage that they could do a film covering Orson's whole adult life, not just 1950's forward.
Oh, and Angus McFayden (sp?) of course.