There is one way to achieve this: first you can constrain your collage to the size you’re printing at. For example if printing at 4×6″ you could select this size from the dropdown on the left (when creating your collage). Then if you have 2 columns you’ll know each will be exactly 2″ wide when printed. If you have 4 columns you’ll know each will be exactly 1″ wide.
Let us know if you have any further questions.