Ship size should also depend on the mission. Wait, mission? Yes, the mission the ship is fulfilling. Because you don't also have to contend with foreign empires.   Even at Trafalga, you had Frigates, Sloops, Third rates, and Second rates. Because you don't just have an enemy fleet to contend with and sending a superheavy battleship of the Deathshadow class after a group of pirates that barely can cobble a CA and CV together with a few dragon and longships makes no sense.

Also, what is the background of the universe you are playing? Even in Star Trek and Star Wars you had the Blockade Runner and the Nebulaon Frigate alongside the Imperator class Star Destroyer.  Who here can forget the Defiant and Miranda class serving alongside the Enterprise and Excelsior class. Why do escorts exist? Because you sometimes need a small ship to do a small job?   A fleet needs eyes and ears to extend its sensor net. Sometimes that requires a ship that can be sacrificed. Anti-piracy patrol. Lastly the bane of us all COST.  What sort of budget do you have? Do you want 5 superships to cover your empire or will two dozen ships half the size be able to respond to threats better?