Well, I've only been trying this ONE time before, so I don't even know
if it really worked (
;D ) or if it was purely coincidential
But here's what I did :
In a custom plane of mine, I added a custom hard mounted machineGun.
Now, in the "cfgWeapons" / "class MyWeapon" -part of the config I wrote :
flashSize=0.200000;...which ( I believe ) would decrease the size compared to, say 0.700000;
Try it out, matey