They dont *have* to be of a specific internal volume. If you mess around with some speaker designer software for a driver which works best in a closed box, you can see the trend the frequency response takes as you make the cabinet larger or smaller.
If in doubt, all you need to do is calculate the internal volume of a production cab based on the speakers you intend to use.
Simple
And all without the horrid splay of gunk coming from the back of the cab
