Depends if you roll it up, if you do roll it up then you can pretty much fit as much as you want.
If you don't, and want to stick the poster on the inside of the box (The actual answer) The longest length it could be depends on if the measurements were taken inside or outside the box. If they were taken inside, you could fit the whole 18 inches long. It taken outside, you could fit 16 inches inside the box since where the sides meet take up about 1 inch total of the inside space.
If this is a homework question, your math teacher needs to be more specific.