Ship Sizes on MoO1:
Huge, Big, Medium, Small and Tiny
Ship Sizes on MoO2:
Doom Star, Titan, Battleship, Cruiser, Destroyer, Corvette
While on Starfarer you have 5 if you include Fighters, this means the Doom Star Class will need to be in the "Battlecruiser" section and Frigates will just have to merge with Destroyers...
Doom Star Description:
The first Doom Star was actually a huge Factory Station on a Meklars world who was about to be attacked by the Antarans, due to the several strike forces send by the Meklars to delay the incoming Antarans fleet, they managed to convert the station into the biggest ship ever made by any race until then, the at that time Station had enough parts to build 4 Titan class ships, were instead used on the station itself, the result was a huge battleship that had enough firepower to destroy almost any battle fleets from any race except the Antarans. Luckily for the Meklars, Antarans didn't know the planet defenses changes in such a short frame time and their strike force was assumed to be more then enough to overcome and destroy the colony. Once they arrived to the planets range, they were almost instantly destroyed by the Doom Star. After the battle, every race started to build a limited number of Doom Star class ships to serve as their main flagships, their number is assumed to represent the strength of each race military fleet.
Titan Description:
While bigger doesn't necessary means better, when the need for more firepower to overcome capital ships comes a bigger ship class comes in, the Titan Class purpose is of a flagship, huge carrier or a heavy battleship, they represent the might of a races fleet until Death Stars started to represent that role, still they are extremely powerful capital ships.
Feel free to post any changes on those descriptions...