I haven't done any myself, though I have had a number of things printed -- business cards, flyers, etc.
Standard Film Poster size is 27x41, and generally movie posters are printed both front and back (image reversed on the back) so that when they are in a back-lit display case the images don't get washed out looking..
Typically with this kind of higher end printing, you'll want images and whatnot to be at 300dpi or higher resolution. Most printers I've dealt with ask for either 300 or 600dpi.