Real low. Not seen outside of institutions, unless you include todd browning's classic ''freaks''.
Rare. True microcephaly (head circumference in the 0.1 percentile, roughly) is rare. A small head secondary to e.g., fetal alcohol syndrome is not so rare (and not nearly so severe). True microcephalics have very severe cognitive deficits so iq measurement is meaningless (it fails at both extremes). With fas, the average iq is about 60.