I don't set more than 500, because by the point you have enough ships to fill your share, game is pretty much won.
Any more is just giving extra advantage to enemy. Well... a potential one, because it would force me to avoid battles with multiple enemy fleets.
For me, that is when the game begins (again), because I enjoy endgame most.
It would be great if I can cram two full-sized endgame fleets (or more in case of extended battles), but that is not possible even with size 500. I probably would not exceed 1000 because 1) UI assumes three digits or less and 2) my old computer has trouble at 1000 if every ship has fighters.
However, there is a point where too big a size hurts the AI because they deploy everything and either player can stall one big ship to drain CR of dozens of ships and maybe see enemies kill themselves with collision damage. I do not know the exact size this becomes a real problem.