The heads I've come up with so far are:
Joseph Wheelwright's "Sleeping Moon" sculpture at Ashmont Station
Antonio Lopez Garcia's "Day and Night" sculptures outside the Mofa
John Wilson's "Eternal Presence" at the National Center of Afro-American Artists
Ralph Helmick's Arthur Fiedler Memorial on the Charles River Esplanade
Can anyone think of any more giant heads in Boston?
All images ganked from the respective linked websites. Please notify me if you would like your photo taken down.