I have a very simple method which I use in GGmod (and the old GritEcon for SE4)
(Via mounts) Component Prices are proportional to the size of the ship.
(Via hull abilities) Maintenance percent is inversely proportional to the size of the ship, squared.
This results in:
- Build time is proportional to size squared.
- Combat effectiveness is proportional to size (weapons & armor).
- Maintenance is flat rate.
Thusly, you can build Thirty 40kt scoutships in one turn at your homeworld.
You can alternatively build one 300kt Dreadnought, but it takes 2-3 turns at the homeworld.
With scouts, you get ~5 times the tonnage in warships per month, but unless they're dying in combat very fast, maintenance costs will soon choke your budget.
With dreadnoughts, you can build as many as you like, but you have to be patient.
The ship sizes you choose to build will depend somewhat on the components you need to fit in (Spaceyards aren't that small), but for the most part, it will depend on how the war or peace is going.
The longer you are at peace, the larger your ships will become; as your maintenance becomes stifling, you can replace small ships with medium ships, medium ships with large ships. (Thus increasing your navy's size without increasing maintenance costs)
When war breaks out, and you start losing ships, you will need to notch down the sizes you're building. Once maintenance is less of an issue due to casualties, smaller ships are very expendable.