A wall poster can be of any size. Because there are so many ways to align a wall poster, they come in various sizes as well. The horizontal measurement of a typical wall is seven inches to thirty-six inches. In between these sizes, there are posters of ten sixteen, twenty, and twenty-seven inches. You will also find these measurements in vertical sizes as well. Even though square posters exist, they are rare compared to rectangular posters. Some modern arts do not follow these traditional size conventions at all. In that case, you will have to use your gut feeling when you select the size.