It can be any dimension as long as you can fit the driver (possible plate amp in the box).
I have always wondered about a minimum distance for the rear wave of a woofer but I have never found any info, I think it wouldnt matter anyways but could not validate that opinion.
There are issues with standing waves when any one dimension gets longer then a 1/4 wavelength of a frequency within an octave of the design.
I use boxnotes.exe to tell me all my possible box issues.
It is not "open-minded" to reject knowledge - Bob Lee