Using a table saw (or circular saw with a fence) cut the plywood to accommodate your poster. Mine is about 22 1/4” wide and 34” long. I wanted to have a perimeter of wood showing around the poster, be sure and measure your poster to make sure you cut your plywood the correct size to allow for the perimeter around the poster.
I left the width of my plywood as is at 24” because it already has the nice straight edges. Then I just measured those perimeters to determine the length I needed my plywood.