Posters come in many different size formats. Some common ones are:
Medium: 18"X24" - This is a common standard size
One standard is the one sheet (27 x 41 inches). There are also posters available in 8 x 10 inches. For films these are often used for promotion and showcasing in the theater lobby. The lobby card (LC) measures 11 x 14 inches.
One of the most popular is the ½ sheet. These measure 22 x 28 inches. It's set on card stock. The billboard size measures 106 x 234 inches. These are set up along roadside billboards.
Other movie posters include the 6 sheet (81 x 81 inches), the three sheet (41 x 81), and the banner (81 x 24). There is also the mini window (11 x 14), the insert (14 x 36) and the lenticular (27 x 41).
The poster sizes mentioned here are standards but don't be surprised if other dimensions are seen. These posters have become very popular and effective as marketing tools. So expect them to be harnessed in different ways as Standard Poster Size, as you can see, often vary.