MIRV warheads are smaller than regular nuclear warheads. The launch platforms for MIRVed nuclear warheads are the same as those for regular nuclear warheads. But on a MIRV, instead of one regular nuclear warhead there are multiple small nuclear warheads.
I'm thinking of things like r36m, but I guess it is used for non-mirv too. Warhead size is irrelevant as they use Mylar radar decoys or something right? Those can appear as any size so I was focusing on the launch vehicle size for monitoring.
Speed may be a differentiator like you originally said, I'm not sure if they need to end up around the same speed anyway for reentry or not (ones targeting high altitude blast for EMP maybe don't need to reentry at all?).
