Welcome to the Ask an Expert forums! You're doing this project, correct?http://www.sciencebuddies.org/science-f ... p075.shtml
After looking at the procedure for this project, the most important thing is that the box is big enough to fit the white paper models (cylinder, crumpled cylinder, etc.), flashlight, and light sensor inside the box. A box that is bigger than listed in the procedure should also work. However, if the box gets too much larger, then the light meter might have a harder time measuring the light that gets bounced off the white paper shapes. A smaller box won't work well for this project.