actually, you could classify dreadnaughts, if you like, as a ship that has a primary, or spinal-mount, weapon.
I wouldn't really call it a size class, though... more like a design philosophy. You could certainly take advantage of the design idea, though, with setting up centerline non-tracking turrets... especially with something that has a low tracking speed and high burst, like a cannon.
a dreadnaught would likely have more burst firepower than a battleship, but would actually be a bit smaller and a lot more maneuverable, in order to take advantage of the lack of tracking and burst capabilities of it's centerline weapons. So in a way, it's size is more likely to be smaller than a battleship.
I personally find the dreadnaught playstyle... a set of centerline cannons for potshotting and stripping shields, and a bunch of 'external' automated chainguns for picking off fighters and stragglers, to be an extremely coherent and enjoyable playstyle.