you will find people here think something that turns at the speed of a snail is a fighter as well, lol
consensus seems to be anywhere between 100 - 1k mass. I can't see anything bigger than that as a fighter due to how they handle, but that doesn't mean you cant go bigger if you want. As far as what size is best? ehhh.. i guess it depends on what they'll be going up against normally, how big your carriers space for them will be, etc. Outside of a bomber or the token heavy fighter 1k is way too big for a carrier-based one IMO, and 5 200 mass ships are better than a single 1k . Like Groovrider said the main thing you want to aim for with a fighter is the damage it can deliver, and even with limited space so long as you have good power systems in place a smaller fighter can do much higher damage than normal with overdrive weapons or specialized ion/piercing weapons, and ultimately