First on the whole ships of the line thing. The reason for it, as I understand it, was that, in order to bring the maximal amount of broadsides to bear, fleets started fighting in a line. A consequence of this is that ships were "paired off" between fleets. Therefore a ship in the line had to match up reasonably well with any enemy ship, otherwise you'd be destroyed without having much effect, and possibly get in the way of other ships following you.
The point I think some other people are getting at is that in star trek ish combat ships don't pair off, they mass fire. Therefore having four weaker ships in the fleet may be better than a single "second rater"
Also there isn't a perfect correlation between size and combat capability, especially in federation ships. With classes like the defiant and steamrunner hitting harder than federation ships much larger than themselves.
Finally in practice the larger sailing ships were poor sailers and vulnerable to inclement weather, but in trek the largest fed ships are often faster and more resiliant to adverse conditions.
Still some ships are better than others, and just for economy it seems they'll tend to make more of the cheaper but potent ships so, I would say, roughly.
1st rate (biggest most powerful ships) Sovereign Class(uprated)
2nd rate (smaller, but similar to 1st rate, few in number) Prometheus, Uprated Galaxy(maybe)
3rd rate. (the primary workhorse warships) Akira, Defiant, Galaxy, Nebula (the Akira perhapse being the best contender for being the "74" as it's got long range sailing abilities, is combat oriented and good at it. The Defiant is a bit more like a super jet fighter in feel)
4th rate (a bit too weak to reliably pair off with an unknown modern enemy ship, but still "big") Ambassador , Griffen
5th rate (Good scout/cruiser/raider frigate types) Intrepid, Steamrunner, Niagra
6th rate. (would be given frigate type work, but not potent enough to be good at it.) Miranda, Freedom, Sabre a bunch of other classes that got just a hint of screentime
war-sloop (Ships that didn't "rate" but could still fight) Oberth, Nova, a number of others.
Then of course you have to decide where you want to put the big ships that are maybe still in use and are outdate, or are really not for fighting but are still pretty big and have some punch.