Actually, while the volume of the speaker box is critical, the individual dimensions are also quite important.
Each dimension of an enclosure has a specific frequency associated with it. This is the frequency whose wavelength is the same as the dimension. And, you can get standing waves at that frequency, and at every multiple of it. This makes the sound at the standing wave frequency (and multiples) somewhat louder than at other frequencies. (Resonant peaks) If your enclosure has two sides the same length, the sound spikes will be worse, and it will be worse yet if the box is in the shape of a cube, because the standing wave will triple amplified. Ideally, the three sides of an enclosure should be at a .621.62 ratio. This will reduce the resonant peaks due to standing waves to a minimum.