Well, remember that missiles have a designated HS associated with their size, which in turn converts to tons. But I seem to recall Steve posting on the Aurora forums that the tonnage isn't necessarily a direct representation of how massive the ships are, or of the volume they displace. But I could be wrong.
Anyway, I almost certainly recall him saying that the default ICBMs were based off of the Minuteman II model of ICBM. The ingame ICBMs are Size-24. In real life, the Minuteman II had a warhead with a yield of 1.2 megatons of TNT; the Minuteman I, which is essentially the same missile with worse electronics (at least for our purposes) weighed 78,000 lb/ 35,300 kg at Earth-normal gravity, was 18.2 meters long, and the first stage was 1.7m in diameter. It had an in-flight velocity of 7km/s and an operational range of 13,000 km.
So essentially that is the same as the Size-24 Conventional ICBM. Keep in mind, however, that the data are skewed because the Minuteman series was, by Aurora standards, laughably slow and short ranged, as it was meant for in-atmosphere strikes only. So an ASM of comparable warhead strength would be much larger, as it would carry the engines, fuel and likely onboard sensors to engage enemies from hundreds of kilometers away.
Of course, this is largely speculation and shouldn't be taken as fact.