Adam, I think it all depends on how you look at it. Modelsplease has a point that you could look at ships in Star Trek by hull size, where the larger the ship the more powerful it would be. But you could you could structure you ship list not so much by hull size but by capabilites and performace. Lets face it, a Defiant is classifed as an "Escort" because it has a sh#t load of firepower and tactical Maneurvering, but it lacks Warp Speed, range (compared too Crusiers), limited Secondary roles, extremely limited crew accomadations, limited sensor/Scanner Abilites, etc.. Although the Defiants are powerful, they must be catagorized as Escorts because they can't cover other roles very well.
This might be the case with many other ships in Starfleet. For example, why is the Constellations being used so widely (with the few that are left) if they are such an old, ageing design barely enough too fullfil the roles of the fleet? A few months ago, I came up with some theories that the Constellations where the answer too replace the ageing Constitutions. I came up with the theory that Starfleet needed reorganized it ship building structure so that it could produce a vast amount of small and large ships with less strain on resources. This theory works on the principle that the shipyards producing Constitutions and other large Crusers be converted too designs that have more main stream components. The Mirandas, Constellations, Locknars, etc. all have similar design perameters. Its possible that Starfleet, with the growing threat of large amounts of small Klingon ships, needed too find a way too make a large amount of ships but also have available ship yards too produce crusiers/capitol ships aswell. I theoriezed that the Constellations are the "Poormans" Crusier of Starfleet in the 23rd Century and that Mirandas/Locknars where the smaller frigates of the capitols. This gives all available Starfleet ship yards the capability of not only building Big Crusers, but also many smaller Frigates at a faster rate and cheaper cost (all ships use many basic space frames). At the time, Constellations might have been a capable vessel, but in the 24th century their capabilites are much more limited. Its very possible that Constelations could be classified as Escort vessels much like Mirandas.
Anyway, what I think you should do Adam for your mod (and again, this is just an opinion) is too not look at the "FF, CL, CA, DN" catagories as discribing Hull size and combat firepower, but (as I had stated above) a way of looking at hull types by mission and capabilites. For example, the FF bracket could be defined as "Escorts" and that all races would be fit into that catagory by a certain critera. The CL bracket could be used catagorized as "Frigates" of races where they have much wider range of capabilites as compared too the "Escorts" but certain classes would be limited compared too some classes in the "Escort" catagory. the CA catagory could be listed as "Crusiers" of races. They would be similar too the "Frigate" Classes, but are almost alwasy bigger and have more stuff then the Frigates. Then finally, the DN catagory could be listed as "Capitol Ship" or "Flag Ship" catagory where almost all ships of this type are the largest that could be built in their time period. The Excelcors, Ambssadors, Galaxies, etc. would be put into this catagory. Also, the beauty of this system is that it can work with all races as each catagory is pretty general for Hull size as its focus is on Mission Role and Capabilites. For example, most Klingon ships would proabably be catagorized as etheir Escorts or Capitol Ships and not many in between. Even in SFB, much of the Klingon Fleet was like this. Good Frigates and Battlecrusiers, but lack of any good Destroyers, DW's, CW's, etc. until the General War. Same could be said about the Gem Hedar, lots of "Escorts" but very few of the other types.
As for your question about Mirandas, I think they could be listed as "Escorts" or "Frigates" in my example above, but I don't know exactly how you want your Mirandas too work in your mod.