Ah, you've just entered the BoP minefield...
The "official" story (from the DS9 Tech Manual, which is at best semi-canon) is that there are only two sizes of Bird-of-Prey: the B'rel, and the K'vort. The B'rel is the original 109-meter scout version, and the K'vort is the newer 327-meter cruiser version. (Incidentally, neither of them has received a FASA-style alphanumeric designation that I'm aware of. The use of such designations in canon sources has been extremely limited, and often--IMO, anyway--wrong.
) This ignores a lot of stuff--most notably the "D12" designation from "Generations" and the fact that the only time in TNG we saw a ship identified as K'vort-class it was in an alternate universe ("Yesterday's Enterprise"), and the only time we saw a ship identified as B'rel-class it was stock footage of the "K'vort-class" from "Yesterday's Enterprise"--but has generally been accepted. It works well enough--somebody, I think it may have been DITL, proved that the whale bay would just fit inside the aft section of a 109-meter Bird-of-Prey--if you ignore a couple of episodes here and there, like the Galaxy-sized "D'Tai's" in "The Defector" and the tiny Hallmark Christmas ornamanet BoP's in "Way of the Warrior," which are only about 35 meters long compared to the Vor'cha IIRC.
As for the "D12" from "Generations," IIRC we never see it in a shot with anything else that would allow us to get a scale reference. Based largely on the fact that the bridge set used is the larger one made for TNG, I prefer to think that the D-12 was an earlier version of the "K'vort," or in other words the 327-meter cruiser. (I also do this because, personally, I prefer to leave the "D" prefix for cruisers, but that's just a personal thing and not really much in the way of evidence.
) Perhaps the D-12 was the first of the 327-meter cruisers, created instead of a new cruiser design because scaling the BoP design was somehow easier given the Klingons' environmental and economic problems after Praxis exploded.
Hope that answers at least some of your questions, MP...