COMMON MOVIE POSTER SIZES
Half Sheet - Half-sheets are 22" x 28" horizontals (half the size of a one-sheet) and were printed on thicker card stock making them more durable than thin paper one-sheets. Half-sheets were usually mailed to theaters rolled in tubes but were sometimes folded.
One Sheet - One-sheets are 27" x 41" verticals. The one-sheet is by far the most widely used movie poster size. In the mid-1980's 27" x 40" (borderless) one-sheets started to appear.
Three Sheet - Three-sheets are 41" x 81" verticals. They are three times the size of a one-sheet. Many three-sheets were printed in two pieces and assembled at the theater for display.
Six Sheet - Six sheets are 81"x81". They come cut in two to four pieces. Six-sheets and three-sheets are most easily displayed if they are linen-mounted.
Insert - Inserts (or "insert cards") are 14"x36" verticals and were printed on thicker card stock making them more durable than thin paper one-sheets. Inserts were used until the 1980's.
The source for this and a whole lot of other incredibly interesting and useful information about movie posters, is learnaboutmovieposters.com