ORIGINAL: Spidey
There's no diminishing return on size in terms of speed or turn rate but there is one in terms of acceleration. A size 400 ship with 14 proton thrusters and 4 thrust vectors will be no faster and turn no faster than a size 800 ship with 28 proton thrusters and 8 thrust vectors but I'm quite sure you'll see the smaller ship accelerate faster than the bigger one.
That is exactly right, Spidey. I tested this with the following ship sizes:
400 size, 14 thrusters, 4 vectors
800 size, 28 thrusters, 8 vectors
1200 size, 42 thrusters, 12 vectors
1600 size, 56 thrusters, 16 vectors
2000 size, 70 thrusters, 20 vectors
These ships were escorts with the minimum necessary equipment and the balance of the weight made up with armour.
In every case the impulse and cruise speed stayed the same. In fact, the cruise speed reduced at 2000 size, but this was because of lack of energy. In all cases, the turning rate was the same. In all cases, the acceleration decreased with increasing size, from a maximum (at size 400) of 13 to minimum (at size 2000) of around 4.5.
One point that surprised me was that you could maintain the acceleration by adding reactors. Thus, if you added an extra reactor to the size 800 ship, the acceleration was the same as the size 400 ship. Same results with adding 2, 3 and 4 extra reactors to the size 1200, 1600 and 2000 ships, respectively. Adding even more reactors would further increase the acceleration till you eventually reach a point at which the extra benefit from the reactor is less than the effect of the weight; at this point acceleration decreases with added reactors. I'm not sure I'd ever previously noticed that adding reactors affects acceleration.
Edit: One thing I would say is that with very large ships I don't think you can achieve the very high levels of acceleration that you can achieve with smaller ships, because diminishing returns kicks in. So, for high acceleration ships, it seems smaller would work.
Osito