Standard Salvo size will be whatever will overload the defence, and in playing with the numbers I think defence in big ships can handle a lot of inbound missiles, assuming that millions are spent in Core computers, turrets and software.
Assume an EW skill of 2, so send 6 missiles for that

EW Software 3 so send 9
Pilot Evade: 2 so send 6
Gunner Skill 2 so send 6 per turret
Assume all turrets at triple turrets giving +2, so send 6 missiles per turret.
Plus 36 missiles for the 2D roll
So 6 + 9 + 6 + 6 + 36: 63 missiles, plus 6 for every turret that might do PD against you.
So your 100 missiles would work pretty well.
The shortened flight times means EW will get rid of best case scenario, 12 (roll) + skill 2 + EW software 3 + pilot evade 2 + anything else: -8 Effect: 11 missiles a turn. That will be thirty-ish missiles in the 4 rounds inbound from very long range. (assuming last round does not get EW roll due to phases. )
Each turret could take out roll 12 + gunner 2 + software 2 + 2 for triple turret+ whatever else I forgot - 8 Effect equals 10 -12 missiles destroyed per turret. So 10 turrets could take out 100 missiles. So to overwhelm a 1000 ton ship send in 150 missiles. Assuming average rolls it will be 7 - 10 missiles a turret, but it shows they can handle a lot of missiles.
Screening ships with software can also EW and PD so it gets even worse. A big ship could have a couple of 1000 ton "turret ships" Designed to take out 100 missiles a turn of incoming missiles each. It depends on the amount of money you are willing to spend.
For small ship combat the 30ish missiles thrown away means that small scale combat will be problematic for missile throwers that cannot throw a lot of missiles. They may be forced to close with an enemy to get to shorter flight time range and then hope they PD does not take out the salvo. An 800 ton mercenary cruiser could be able to PD 40 - 60 missiles after a single EW attempt takes out 10 missiles. So a small ship will need to throw over 80 missiles to hope to get anything through.