So in trek, frigate<destroyer<cruiser<dreadnought?
Is it just about size and engine power then, relative to peer ships? There's no distinct division or distinguishing features.
This is not so much a matter of just size/power, as it is of overall configuration.
Generally speaking, a cruiser is a powerful multifunctional vessel. It's not the most powerful in combat, nor does it have the best "scouting/recon" capability, nor does it have the greatest cargo capacity, nor is it the fastest. It is the "jack of all trades" ship, and though it's not the BEST at any one function, it's better than anything other than a dedicated-role vessel in fulfilling any given function. It's what you'd send to do a job if you weren't QUITE sure what you were going to be encountering (combat? exploration? running away really fast?)
A frigate is a step down from a cruiser, but is sort of the same thing... really a multi-role vessel. Frigates are usually the real "workhorses" of the fleet, doing all the "routine" work that it's not worthwhile to send the far more expensive Cruisers to do. It's fast (potentially faster than a cruiser, but if so, not by much), well-armed, has good recon capability, has reasonable cargo space, etc... but is much cheaper than a cruiser and as such is far more numerous.
In Trek terms, the Reliant would be a frigate while the Enterprise would be a cruiser.
Pretty much everything else are "master of one" type designs. Dreadnoughts are essentially a subcategory of battleships... big, very well-armed, extremely expensive, and not particularly well-suited for non-combat, peacetime jobs. (A dreadnought, in real (historical or modern) navy terms, is simply a battleship with a particular quirk of weapon... all the main guns being the same size.)
In Trek terms, a dreadnought would be considered either a big "show of the flag" or, more likely, a "saber-rattling" display, during peacetime. I like the idea that these are normally assigned to stationary duty at Starbases, providing the core of the local security force and also doing limited localized "peacetime" jobs, but during wartime they'd become mobile command posts - the center of each major battle element.
Destroyers aren't necessarily smaller, slower, weaker, or less well-armed than cruisers or frigates. In fact, some may be smaller than frigates and some may be larger than cruisers. But they are basically dedicated weapons platforms without any other role. A destroyer's job is to shoot, and to survive while doing so. In peacetime, you'd probably have destroyers doing border patrols (not being the primary patrol element, though... rather, being the "reinforcement" that the smaller patrol vessels would call for help!) or patrolling spacelanes. They COULD do peacetime roles, but wouldn't be very good at it (with very limited built-in capability for such roles).
Scouts are generally going to be heavy on sensor and "stand-off" science capabilities. Their job isn't to fight (though they should have the ability to defend themselves to some extent!). Ideally, they'd never come into enemy contact at all, and if they did, they'd be fast, light, and stealthy enough to evade their attacker. The rule would be "run away if engaged." During peacetime, they would also likely be given patrol duty in fairly secure, interior regions, and well as basic mapping/exploration duties.
Then you get into even MORE specialized designs. Dedicated science vessels (which are sitting ducks without an escort), like the Grissom. Big cargo-haulers. Service vessels (essentially the spaceborne equivalent of AAA - if you break down and need service in space, this ship can fix you up and get you home!). Shuttlecarriers, passenger carriers, mining vessels... all of which would be dependent on other ships (or in the case of the carrier, possibly its own embarked vessels?) for their security and support needs, but not really intended to ever operate in "unsafe" areas without an escort.
That's how I see it. It's not about SIZE, it's about DESIGN INTENT.
