Historically, wet navies used the ship's role for classification, not its size (although, since the ship's role also dictates its size, there is a significant correlation there, if not true causation).

Carriers "carry" fighters. They tend to be really big because of economy of scale: it's much easier logistically to maintain one ship with 10 fighter flights than 10 ships with one flight each. (In the real world, carriers are big because of the need for a flight deck; in space, such things are unnecessary, so you *could* have the lots of ships with a few fighters each, but sci-fi settings tend to portray carriers as being as large as, if not larger than, battleships.)

Battleships (short for "line of battle ships") are those meant to stand in the line of battle and kick the crap out of the other guy's battleships. Thus, they tend to be the largest ships in the fleet, because bigger is better. Some settings use "dreadnought" as a classification for "heavy battleship" -- while, confusingly, others use it as something lighter than a battleship. It is true that battleships (and battlecruisers, for that matter) were called "dreadnoughts" after 1906 (specifically, "Dreadnought battleships"), but within 10 years these were superseded by super-dreadnoughts, and since "super-dreadnought battleship" sounds silly (and there were no non-dreadnought BBs anymore) the term "battleship" was back on its own. In the end, a battleship is a battleship is a battleship: the biggest, baddest ships in your fleet.

Battlecruisers were an experiment by WW1-era navies, trying to combine the offensive firepower of a battleship with the speed and independence of cruisers. Generally considered a failure, if unfairly so (IMHO). The term fell out of favor by WW2, with only a few examples from that conflict and the Cold War, all of which can be described as "cruiser killers" -- ships with big guns able to defeat any cruiser they come across, but without the defenses needed to go toe-to-toe with a battleship. Sci-fi settings typically consider a battlecruiser to be a "light battleship" (or a really nasty cruiser), with little consideration for this nuance.

In the wet-navy environment, cruisers are meant to "cruise" around the oceans, conducting long-range scouting, engaging in commerce raiding, showing the flag, etc. They generally act independently, and need to be big enough to look after themselves, while fast enough to run away if needed. They can't be too big, though, because at some point they become too valuable to risk on such missions.

Originally there were such things as simple "cruisers"; however, they were unarmored. Once someone thought of putting armor on the deck (to protect the vital spaces below), these became "protected cruisers"; those with armored belts became "armored cruisers". Protected cruisers eventually disappeared as technology improved. Armored cruisers were redundant after battlecruisers came on the scene, and when battlecruisers fell out of favor, there was no impetus to go back to them.

In WW1, there were (obsolete) armored cruisers and light cruisers. Both were used in traditional cruiser roles, while armored cruisers sometimes went up against capital ships, with catastrophic results. Light cruisers also found use as leaders for destroyer flotillas. Between the wars, the Washington and London naval treaties created space for two classes of cruiser: light cruisers had guns of less than 8", while heavy cruisers had guns 8" or larger. There are instances in which "light" cruisers have a greater displacement than "heavy" cruisers. However, in sci-fi settings, light and heavy refer without exception not to the class of armament, but relative size.

Destroyers were originally "torpedo boat destroyers", designed to fend off enemy torpedo boats and keep them away from the battleships. In an ironic twist, they began carrying torpedoes themselves and eventually took on the role of what they were meant to defend against while retaining a secondary defensive purpose. By WW2, destroyers took on many forms, all lumped into the general category of defensive escort for (and potential threat to enemy) capital and merchant ships -- with various anti-aircraft, anti-sub, and anti-destroyer roles. On the whole, you need a lot of them, and they need to be expendable., so they end up small and cheap. Modern destroyers are larger and have drifted into traditional cruiser territory, giving up their defensive escort role to frigates.

When it comes to sci-fi, no ship classification is so confused as the "destroyer". You've got the wet-navy paradigm of a small escort ship, to the Omega-class destroyers of Babylon 5, which are basically heavy cruisers, if not small battleships, all the way up to the Star Destroyers of Star Wars, which refers to an entire range of ship designs which are clearly the most powerful in existence.

Initially defined as a fast ship in the 17th century, a frigate wasn't of any particular size: for the most part, they weren't meant to stand in the line of battle, but some fast battleships were considered "frigates". By the later age of sail, frigates were essentially large cruisers -- as long as ships of the line, but faster and more lightly armed. Then, around the ACW, the "armored frigate" was introduced, and for a short period these were the most powerful ships afloat. These eventually morphed into battleships and armored cruisers, while the term "frigate" fell out of use. Then, in WW2, the British brought term back to describe an ASW escort smaller than a destroyer. Sci-fi settings generally consider the frigate to be a "small destroyer".

That's my point -- traditionally, there is no "range" of hull sizes (i.e. displacement) within which a destroyer, cruiser, battleship, etc. would be defined as such. It's all about the role the ship is meant to play in the fleet.

But I also realize that many (most?) starship combat games set up such ranges, so I understand the question. Suffice to say, if I were to provide an "official" list, you and others on this forum would chafe at the size constraints.

Your Unity Guidelines are similar to what I use; I classify my OKLAHOMA design as a BB: :geek:

SNS OKLAHOMA  Battleship (700)
-Tech: Engines +1

Armor: 18-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1
Hull: 18-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1
Engines: 4-4-4-3-3-2-2-1-1
Weapons: 23-21-18-16-13-11-8-6-3
Shields: 3-3-3-2-2-2-1-1-1

14" Plasma Guns  (1-6/7-12/13-18) 1×3+/1/4
ABCD3 ☐   ABCD2 ☐   CDEF2 ☐   CDEF3 ☐ // (2)

CIWS (0/1/1) 3×2+/1/1 (Dfn)
360° ☐   360° ☐   360° ☐   360° ☐   360° ☐ // (3)

Equipment: Hyperdrive ☐| Overthrusters ☐| Tractor Beam ☐☐☐☐☐☐☐☐☐☐☐☐ // (7)
Traits: Damage Control <4>; Cargo (3); Repair (2)

BTW, SNS=Steven's Naval Ship
Cheers
Steven

Now I'm trying to recall, didn't you once make a comment regarding size-creep during the Nova days?

I realize that 18 hull & armor is large, but it is the smallest I could make this ship given these criteria:   :ugeek:
Only one +1 in any tech level,
Triple & twin turrets to match what USS Nevada actually had,
Good damage value for the 14"Plasma Guns, 
Significant AA fire (average of 18 fighters destroyed per turn)
Engines = 4,
Adequate shields = 3.

The largest Naval BB design is 20 hull & 20 armor, my SNS South Dakota design.  We often play battles with 3,000+ points a side.
We are playing tomorrow and the SNS Nevada will be in my force...
Cheers