When your poster is not bound your cell sizes can be set to specific sizes. When you want to add a cell click File|New Cell from the menu. You will then see the above dialog. The options allow for you to create a cell as well as taking into consideration the fact that this photo may be appearing behind a photo mat. In the example above we are creating a 3"x4" photo based on the opening in a mat. In the overlap field we add 1/8" overlap setting to compensate for any slipping from the photo being under the mat so you don't see any blank paper behind at the edges of the mat opening.
If you weren't going to put this photo in a mat and you wanted an exact 3x4 photo you would set the overlay setting to 0.00" so it does not increase the size of the cell.