Re: 15" speakers????
The two most important variations in speaker driver charateristics with size are efficency and frequency response at the top.
The efficiency (louder sound for the same number of watts) increases with size, in general.
The top end gets worse with increasing size, because the resonant frequency decreases (in general), and because of some complicated propagation effects. (This is the resonance at the top end that matters here; there is one at the bottom end as well that sets the lowest useful frequency.)
12" really seems to be the best compromise for guitar. The resonance sets the guitar voicing and gives very high efficiency in the frequency range where it is needed most.
The somewhat lower resonance of the 15" is great for most bass applications, but it might not give enough "snap" for some styles.