I meant to respond to this interesting question earlier, but real life has had a way of distracting me lately
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In the 6 Powers Campaign, the offensive missile sizes range from 3 for the Japanese Long Lance missile line, to larger for the Reich and the Alliance. I can't remember the exact size for the Reich and the Alliance right now, but they are larger, and the Alliance has introduced an even larger missile lately, because they needed something bigger to fit in ECM.
Having said that, I am undecided as to the "Best" size. The Japanese have a good salvo size and ROF because of the relatively small missiles, but each missile only does 3 points of damage, so even though they launch a lot of missiles, and usually get at least a moderate number of hits, they don't do much damage.
Some things to consider here are ROF, and the balance between capability to sustain a continuing engagement vs being able to overwhelm an opponent right off the bat. The Reich Scharnhorst missile frigates use box launchers and can launch 20 missiles at once, which isn't bad for a 3,500 ton ship (approximately). In a same-size match, three of these could launch sixty missiles in one salvo at an Alliance Battlecruiser or Dreadnought, completely overwhelming its defenses and scoring multiple hits, probably enough to destroy it, or at least seriously damaging it. However, once they have launched their missiles they are nothing but targets, and are useless until they are reloaded.
Kurt