My fighters are 1000 tons, anything less is a missile. The list below is what I generally use. Though don't be surprised if I break my own rules. It all depends on how I see the government/people I'm playing as.
Gunboats 1,000-5,000
Corvette 5,000-12,000
Frigate 12,000-20,000
Light Destroyer 20000-25000
Destroyer 25,000-40,000
Light Cruiser 40,000-60,000
Cruiser 60,000-80,000
Battecruiser 80,000-100,000 (Also requires the speed of a destroyer)
Battleship & Escort Carriers 100,000-500,000
Heavy Battleship & Carriers 500,000-1,000,000
Dreadnought & Assault Carriers - 1,000,000+
Super-dreadnought & Fleet Command Carriers - 12,500,000+
Monitor & Super Carriers - 25,000,000+
Super Monitor - 50,000,000+
Star Base - 500,000,000+
Super-dreadnoughts and above are increasingly infeasible from a logistics standpoint.
Such a project wouldn't be done for the weapons or firepower, it would be done as a government mega-project to awe the civilians and show the might of the Emperor (Or council, king, queen, etc.)
It is incredibly hard to build an maintaining them is even worse. Do you know how many maintenance facilities it would take for a Super Monitor to overhaul it?
Also if the maintenance failure rate gets high enough it overflows and no maintenance is needed which I imagine will need to be fixed soon =)