If you are using Photoshop Elements, the Photo Collage Wizard only has limited size choices with no ability to make customized changes.
However, you can make post-Wizard changes. Once you have clicked Done, you will be asked to save the collage and it will close. After you have saved it, reopen it. This will open the file outside the Wizard and allow full editing capabilities. Just go to Image>Resize>Image Size.