For bass and pro audio enclosures the volume is very crucial, as bass and pro audio speakers are more cabinet dependent. It‚Äôs not as crucial for guitar speakers because 1.) guitar speakers have rather odd parameters (high resonant frequencies, high Q values, and low Vas), 2.) guitar tone is very subjective, 3.) guitar speakers don‚Äôt have to play as low in frequency, and 4.) guitar speakers are used to create tone, rather than just reproduce sound. The wood type of the cab and the tonal characteristics of the speakers are much more important to focus on than the dimensions. I generally recommend building to similar dimensions of a cab you have heard before and that you know sounds good. Also keep in mind that a larger cabinet volume will promote more low end. A smaller cabinet won‚Äôt provide as much low end, but may sound tighter and punchier.
Also worth noting, in choosing an open or closed back approach, decide if you want a more focused sound with enhanced bass response (closed back) or a more room filling, and for lack of a better word, a more ‚Äúopen‚Äù sound (open back).