1. Interstellar Arms Limitations - who'd enforce it? Also, trust and verify part would be impossible.
2. About all you could do would be make an agreement would be where to place a Neutral Zone, and what could violate it.
Who'd enforce it? About the same as now: when they saw advantage in an agreement, they'd make an agreement, monitor it, and keep it to the extent that it was in their interest to do so. When they no longer saw an advantage in an agreement, the agreement would be scrapped. So long as your incentive to keep the other guy honoring his end was high enough, you'd mostly honor your end, and so long as his incentive to keep you honoring your end was high enough, he'd mostly honor his end. There isn't any need for enforcement as the agreement only exists while it is mutually beneficial.
That's the way it worked with the Washington Treaty: the signatories signed on because they saw an advantage in the agreement. There was some cheating and some stretching of interpretations, but they generally adhered to it because it served their own needs to do so. Eventually, one party backed out of it when they felt there was no longer sufficient advantage.
The real question isn't who will enforce it; the real question is whether the opposing sides see advantage in coming to agreement on a limit, and whether - having accepted a limit - they continue to see advantage in respecting that limit. The larger the empire - the more ships they have, and therefore the less the fate of any single ship matters to the overall war effort - the easier it becomes to agree to terms that set limits on the power of an individual ship. Similarly, the higher the tech level involved, the easier it becomes to set and adhere to limits because the increasing power of nuclear dampers makes missiles less useful.
Put another way: you have a problem - an arbitrary rule. You have three choices. You can house-rule away the problem, but then you're out of step with others who use the rule. You can wave your hands while giving some techno-mechanical mumbo-jumbo explanation for the canon rule, but no one's actually gonna buy the explanation. Or, you can lay the blame on sociopolitical forces and say it's because of some old treaty. The advantage there is that you as game master are the sole arbiter of what is or is not possible in your setting's political landscape - no one can argue, at least not the way they'd argue if it were some science or engineering based rationalization - and there's just enough historical precedent there for you to say it's at least feasible.
A sociopolitical rationalization leaves open the option for the game master to keep the bulk of canon ship design while introducing house rules that depart from the rule - pocket empires that refused to be bound by such treaties, pirates who modify their ships, earlier eras when ships were built by different rules and so forth. You'd need to make up those house rules: the computer DM mechanic implies that some feature of the launching ship remains a bottleneck at least to terminal range, if not to impact, so it's likely you'd still be restricted to one launched missile active per launcher - none of this "cargo bay dumps a thousand missiles to space" business. However, it'd be very interesting to design warships that ignored the rule and test them against warships that followed the rule. My guess is it'd make the biggest difference in the smallest and lowest tech ships and progressively less difference as you progressed to larger and more advanced ships.
As for "verify", we're talking about sci-fi interstellar empires here: spy verses spy, brave Bothans dying to bring you information, scouts mining ice so they can sneak about in the distant hinterlands of a system spying on its central worlds, ships with densitometers scanning you down to your deck plans as you cruise by. Goodness knows, it'd still be difficult - the other guy is doing his level best to keep his secrets - but where there's a will and the resources to find them out, there's a way. It's only impossible if you want it to be impossible in your own campaign setting.
