Until the late 80's, movie posters were printed in a number of sizes.
11" x 14" Lobby Card 14" x 22" Window Card
14" x 36" Insert 22" x 28" Half Sheet 30's" x 40's" 27" x 41" One Sheet
40's x 60's 41" x 81" Three Sheet 72" x 72" Six Sheet
Billboard size Twenty-Four Sheet
Since the mid-80's, these sizes have been gradually phased out to just one main size: the 27"x40 One Sheet. Lobby Cards were generally printed 8 to a set, with each card showing a different scene from the film. In some cases, Lobby Cards may have been printed for the 1990's titles, but these are not plentiful and may not have been printed at all.
Virtually all movie posters have been copied, so it's critical that one purchases an "Authentic" item. Since 99 % of my collection has come from theatres, you can buy with confidence. I guarantee that all my posters are authentic.
RE-ISSUES: Many popular films were so successful, they were "re-issued" again in theatres. Disney is especially known for re-issuing their classics. Re-issues generally have an "R" in the bottom right corner of the poster, as in R56/101. The number after the slash is the one-hundred-first film released during the year. It does NOT mean there were 101 posters printed. The lower the number, the earlier in the year was the release date.
FOLDED OR ROLLED: Prior to the mid 80's, it was common for posters, especially One-Sheets, to be folded as they were shipped to the theatres. This is also true of the other sizes, except for Window Cards, Inserts, Half Sheets, 30's x 40's and 40's x 60's which were printed on heavier paper stock. These shipped rolled. Beginning in the mid 80's, most titles, especially One-Sheets, were shipped rolled by the film companies to the theatres. However, it is not unusual to find some folded One Sheets from this period. Today, virtually all One Sheets should be rolled or unfolded.