[Taz-in-Space]

I have extensively used fighters and still do. As to best fighter group size, that depends on what you do with them and your opponent's ship configuration.

Small fighter Groups have the advantage of having the opponent 'waste' firepower as previously mentioned. Also in tactical battle mode you can use fighters to block movement of your enemy's ships. You can even stop or isolate them by surrounding them with fighters. I sometimes use large numbers of small fighter Groups to board enemy ships - fighters surround the ship and take down the shields and a 'taker' boarding ship then nips in to capture the enemy ship.
If I want to just kill ships, I use larger Groups of fighters for the firepower.