Small: 11×17 Poster Size
11×17 inches is generally the smallest poster size. Anything smaller won’t look like a poster, but instead like a piece of A4 printer paper tacked up on a wall.
Medium: 18×24 Poster Size
18×24 inches makes for a medium-sized poster that can be displayed both indoors and outdoors. 18×24 is an appropriate and normal poster size for just about any project because it can be appreciated both close up and from a distance. This is also a regular poster size for framing, so you have many display options for 18×24 posters.
Large: 24×36 Poster Size
24×36 inches are the dimensions in which large format posters are most often printed. If you were to go into a store that specifically sold posters, chances are a large portion of their inventory would be 24×36 posters. You will find many options for framing posters that measure 24×36, and with an aspect ratio of 2:3, many images (including movie posters) fit nicely at these dimensions.