Magazine capacity should be at least 20 salvos, 40 is better.
Don't worry about reduced size launchers for this class unless you go for box launchers, the reduced mass for greater numbers usually isn't worth the increased reload time. At least 10 launchers for the broadside is a good start, more is of course better since you’ve have to overwhelm the targets missiles defenses.
This is quite good, except for the above bit.
I find reduced size launchers to be extremely broken. Currently, I'm packing 33% reduction launchers and when I have good enough reload rate tech, I'll swap to 25% launchers.
Generally, I time my reload rate to around a quarter the missile flight time at max range, give or take quite alot since you can't finetune it. But I make sure to always have the launchers reload faster than the missiles fly at half range.
This allows a puny 6kton frigate to throw a 24 missile broadside. Of size 5 missiles. My 100kton fleet has a total missile salvo of over 1000, with a third of my ships dedicated to missile defence.
I'm seriously considering downsizing my missiles to size 3 or so as this will let me pack 25% launchers...
Against a fleet that is smaller than half my fleet's tonnage, my fleet can vapourize it in a single salvo if they have expected armour levels (~5 or 6 say). Repeated engagements against the wormhole aliens have let me estimate the salvo size I need to blow them away and they haven't so much as scratched the paint despite me packing no shields and having slower ships than them.
Against a similar sized fleet, my fleet can smash all the important targets (like fleet scouts and jumpships) right through the thickest anti-missile defence. A full salvo, which I haven't got to use, from reduced sized launchers is basically unstoppable.
Of course, my fleet only has about 3 salvoes or so in ammunition but hey, it can blow itself into scrap in 3 salvoes, so that's fine right?
Of course, this doctrine relies on me scoring the strategic advantage so if I don't have that... >.< Guess I'll be handing out alot of posthumous medals.