Over time, the 12-inch speaker has come to be regarded as having the best balance of attributes such as low end, harmonic complexity, power handling and high-note dispersion. However, there are some great sounding 10s and 15s that can offer some alternative, interesting and even exotic flavors.
Good quality 10s can deliver a fast, punchy sound at wider listening angles with reduced ‘boom’ on small stages. They can offer increased portability, reduced cost and the ability to push your amp into overdrive at reasonable levels without having drumsticks aimed at the back of your head.A well-designed 15 can move more air so you can gig those wonderful little valve amps. The vocal range can be creamier, with extended low end and lots of detail and harmonic complexity, giving surprising richness to some otherwise scratchy-sounding guitar and amp combinations.
For more background info, check out this article for more detail about size and the loudspeaker design process.