How small is small?

Fair question. I'd expect an LW post & discussion of similar size to this one to cause 10^-3 to 10^-4 suicides, but I might now be anchoring on your reply. (Also, that expected value only counts one side of the ledger; I'm ignoring the possibility of the discussion discouraging people from committing suicide.)

If the chance would be p=0.001 that the post kills someone, is that small?

To me, it feels big, in the sense of disproportionate, but small in absolute terms.