If they're domesticating themselves in some way, natural evolution can take care of the rest (e.g. smaller sizes leading to long-enough lifespans to successfully mate and raise offspring long enough to repeat the cycle).
“Palaeoloxodon falconeri is an extinct species of dwarf elephant from the Middle Pleistocene of Sicily and Malta. It is amongst the smallest of all dwarf elephants at only one metre in height. A member of the genus Palaeoloxodon, it derived from a population of the mainland European straight-tusked elephant (Palaeoloxodon antiquus)”
