Like most people I use flexible tonnage, classes will creep heavier over time, but generally they'll stay in a similar ratio to each other.
Apart from using a size hierarchy, ships are usually differentiated by function too.
Generally for ships of the line, those intended to travel at standard speed and with sufficient range and flexability to operate independently by size they increase in size thusly:
Corvette, Frigate, Destroyer, Cruiser, Dreadnaught.
Generally I'll try to use the same engine for as many ships as possible, so the Destroyer might use a single engine, cruiser twice the size shall use 2, Dreadnaught using 4 or more.
Battleships are generally cruiser size, or more. They differ from cruisers in being heavily armoured and designed for breakthrough. Generally all beam or short ranged high damage missiles, Battleships may generally be slower and shorter ranged than the rest of the line in order to maximise lethality.
Other specialised ship types include;
LAC: any ship under 1000 tons not intended to be launched from a carrier. Usually lightly armoured and short ranged
FAC: similar to above but with speed being maximised, basically a large fighter.
Monitor: short ranged system defence ship basically a large FAC or LAC. Usually Frigate or Destroyer sized but no larger to save on needed maintenance facilities.
My carriers are generally as large as I can field, anything smaller becomes a light carrier, which usually has less dedicated defences which are left to escorts. A light carrier should be fairly equilivent in ability as a standard or heavy carrier as long as a few destroyers tag along.