Can I get a rundown of the whole frigate/cruiser/destroyer/corvette/dreadnought/etc role/size thing please? I can't find it anywhere.
There's no official standard, though there might be a general consensus (~IE, 30,000 tons is battleship range).
Personally, I like to have classes based on the role they play rather than size, as size tends to be flexible as the game advances. So, say, a fairly hefty ship designed for heavy firepower would always be a battlecruiser, regardless of if it's 10,000 tons or 40,000; a heavily armored command ship would be a battleship, a ship designed to provide anti-missile/anti-fighter point defense would be a frigate, etc.
A full list of classifications I've been known to use:
Frigate: Lightly armored, cheap, small (relative to the other ships) ships that use most of their tonnage on point defense systems, and aren't meant to operate on their own.
Destroyer: Fast short ranged ship with an emphasis on armor and firepower, designed to operate in squadrons
Destroyer Leader: Similar to the destroyer, but mounting better sensors and armor but fewer weapons.
Cruiser: General purpose ships, basically; medium size with a balance of everything (range, armor, weapons, etc) but less speed.
Heavy Cruiser: Though the AI very seldom defends warp points, I use heavy cruisers as a nominal assault ship class, with heavy armor and beam weapons designed to be the first through a hostile jump point.
Bombardment Cruiser: Cruisers with very large size missile launchers; I use them for either long range bombardment or as mine layers.
Battlecruiser: The big guns of a fleet, designed to sacrifice just about everything except armor in favor of big anti ship guns or missiles.
Battleship: Heavily armored sensor platform, with a flag bridge if I'm using them. Usually sacrifices some guns and has a few battlecruisers in its traskgroup instead.
Dreadnought: Probably the most size specific; jump drives become exponentially more expensive for larger ships, so "Dreadnought" is my term for a ship too large for a practical jump drive, limited to a jump gate network. Huge system defense ships, basically.