Around since 1909, the One Sheet is probably the most widely used film poster format as it is predominantly used in the US. Until 1984, One Sheets used to be 27 x 41", however they now tend to be a standard 27 x 40" size (with no border). One Sheets come rolled or folded and the portrait format can really enhance the poster art.
7 of my favourites are on this page, however to see my full collection, either browse using the letters from the menu on the left, or click the 'Random Poster Browser' button on the left.