That depends from whom you are buying. If it is unstrung, it means it is 16" without the clasp, when buying the finished strand, very often the clasp is already included when giving the length.
In PP:s case I think it is the real length of the pearls only, but I could be wrong. If not certain, ask before ordering it.
Even with us it is always the length of the entire strand. This is industry standard. If it were any different the type and size of clasp would affect the way a strand drapes. This would create all sorts of issues for a seller.