I tend to agree with Snowdragon.
Assuming all ships can generate power equally efficiently (why I brought that topic up before, BTW), a ship 10x the mass of your fighter has 10x the power. It can engage 10 fighters just as well as the 1 fighter could engage 1 fighter. If you're using missiles instead of energy weapons, it could hold 10x more of them and have the same proportion of room available for other things inside. Theoretically, it can even have nearly-identical handling properties (speed, accel, endurance, maneuverability) as the fighter without sacrificing anything (cube/square stuff comes up eventually, but at that point you can always sacrifice extreme maneuverability and mount more guns/armor/shields instead).
The kicker, though, is that it has 10x the shielding/armor as the fighter, in a package likely not more than 3x the volume. It presents only a slightly
larger signature for a much
stronger vessel. Now blow that up to a ship 100x the mass of the fighter, or 1,000x; or the
~8,300x or so that the displacement of a real aircraft carrier is to a fully-loaded combat aircraft (Gah, long tons).
Now, at some point, it begins to be moot what size your ships are - if your fighter weapons can one-shot that aircraft carrier (without shields and using only real stuff, they could today
if we weren't so concerned with fallout), smaller ships make sense. But then, everyone will be in smaller ships, and large ships wouldn't be a thing. I suppose if you only provide so many weapon types in-game, and some of them are good for somethings and others for others (ala battleship craft, where triple-46'ers can
one-hit fighters but rarely do
), you could create a contrived situation where fighters make more sense (fixed smallish guns that are effective against all ships don't make much sense on large ships, where turning to engage multiple targets is annoying; and limiting turrets to ineffectiveness against fighters except for a few token PD weapons that aren't as long-range as the fighter guns).
Basically, if the game is free-design, pick a size you want to be most effective and design for that; that's what everyone will use.
And while figuring out that long tons are, in fact, different from regular tons; I learned that the real world has tried to deal with this size problem in an interesting fashion (besides being somewhat more restricted by physics): The Washington Naval Treaty
. IRL, the treaty limited the size of each class and the number of each class any signing nation (all the ones that mattered, at the time) could build. In game, there could be an arbitrary NPC police force enforcing the 'treaty' such that, if you build a ship larger than the treaty-size (or your permitted treaty-size), they come after you.