In a nutshell, the theory is that as the frequencies reproduced get higher, the size of the speaker driver should decrease, for best results. As with all things, we have to compromise. I've got a thread running elsewhere about a proposed rig which in effect may be a four way rig. It's impractical for use on the road, it's just a bit of fun to see what's possible with the kit I have.
The " best " results come from a stack of cabs such as we see on Speakerplans and the like, with Subs, Mids, High Mids and tops, for example. The compromise comes, when you've only got a small vehicle to shift them in, and it's a fifty yard carry into a Golf Club ( where the function rooms are invariably upstairs, and lifts are rare ), then that's totally and utterly unfeasible.
My advice would be that if you regularly do functions where speech/vocals are a major part of the event ( Karaoke, presentations and the like ) and you're not looking to go down the line array route ( Bose, Vertus, Maui et al ) then 12" tops ( preferably with subs ) are almost mandatory, and be prepared to lose some bass if you have to use them on their own. ( And again, having said that, John did a gig with poor access, where I refused to carry a powered sub upstairs, and his 12" RCF's on their own sounded lovely, way better than they had a right to.
If you never do Karaoke, and rarely do presentations, commentaries on Sports days and suchlike, then 15" is the way forward, because with or without subs they'll offer a more rounded sound than 12" models of the same cab. ( Usually. ) I just know someone's going to relate their experience of smaller cabs sounding bassier than their larger brethren, ( and I reckon I know who, and what cabs.
) but I'm generalising. If I'm limited to using one pair of cabs for whatever reason, I usually choose the 15" ones.
That's a gross over- simplification, but as always, the advice of "listen before you buy " is worth following.