How much of a thing there is, measured against a standard. First a ruling handed down - the amount an authority fixed as right.
Old French sise, from Latin assisa (a session, a decree), from assidere (to sit beside, to assess) — the same root as assize; size was first a ruling handed down, the amount an authority fixed as right.
How much of a thing there is, measured against a standard someone else first fixed as the right amount.