Posters come in a variety of sizes, so there is no single set of dimensions that works for all. The most important consideration is to use artwork with the highest level of resolution, and that will scale well when expanded. If you have designs that span the full poster, we recommend you use artwork that will match the dimensions of the poster. For example, 24x36 posters at 300 DPI can have an artwork size of maximum 7200x10800px.
Also keep in mind the aspect ratios are different for poster sizes. 18x24’’ posters have a 3x4 aspect ratio while 24x36’’ posters have a 2x3 ratio
For best results, use artwork that does not have any transparent padding around the top/bottom/left/right edges. Any padding on the sides of your main artwork can risk decentering the image during printing, so go edge to edge!