In the section on Traveling by Air (X20), it actually gives guidelines for how big (in HD) a creature needs to be to pick up or carry a person. This is useful not only for transport, but for whether a monster can snatch up a PC and carry him off into the air. For example, 3+ HD is required to lift a halfling or smaller. Therefore, technically, a cockatrice could carry a halfling or gnome off. Weird!
This information is incredibly handy, IMO, as it gives me as a DM a quick reference to decide about the tactics of aerial encounters, which can be complex enough as it is. For example, my group recently fought some harpies. While there were no hobbit-sized PCs or NPCs, it would have been nice to know ahead of time that they could have been carried off. (The idea of a charmed halfling just holding out his arms and being flown to the nest for devouring makes me laugh.)
