Depends, I suppose, on how far down the population gets reduced. The only example we have of human speciation is whatever caused the split between
Homo sapiens and
H. neanderthalensis, assuming they had a recent common ancestor. My guess is that it would require reduction to the point where you have isolated populations on each continent that have lost so much of the cushioning effect of "civilization" (for lack of a better term) that thousands of generations pass before reconnection occurs (if any). Especially given the fact that any surviving population of modern humans is likely going to have a pretty wide cross-section of the available human alleles due to the current effects of globalization.
However, there are a couple of other factors to also consider in any such scenario. The most obvious is at what level would our species become non-viable. I don't know if anyone has done any modeling on a Minimum Viable Population for
H. sapiens. If there is any correlation between us and, say, bonobos, then an MVP
1000 would be 5-10,000 individuals in a freely inter-breeding population. Or more. Some models suggest long-term viability (i.e., greater than MVP
1000) would require 50,000 or more individuals in the starting population due to potential loss of alleles, etc.
Another factor that might be important when we consider
H. sapiens is that our species seems to have something of a wanderlust. IOW, given that it appears there have been at least one and possibly two "out of Africa" migrations in our lineage, it strikes me as entirely possible that there wouldn't be sufficient time to allow isolation and natural selection to "do their thing" before gene flow between catastrophically isolated populations is re-established.
The science here seems to be pretty soft, if you ask me. I'm not sure how realistic any of the above might be when you look at our species. It just might be that the only realistic scenario for human speciation would be the one where utterly different selection pressures operate on a totally or mostly isolated population for a really really long time (i.e., the "other planet" scenario).