Size
Middle English (also in the sense ‘assize, ordinance fixing a rate of payment’): from Old French sise, from assise ‘ordinance’, or a shortening of assize.
wiktionary
From Middle English syse, sise(“regulation, control, limit”), from Old French cise, sise, aphetism of assise(“assize”). Displaced native Middle English grete, grette(“size”) (from Old English grīetu, grȳtu(“size, greatness”)).
From Middle English syse, [1] of unclear origin; [2] related to Old Italian sisa(“a glue used by painters”), [3] perhaps ultimately related to size / syse(“amount”), [2] [3] or perhaps shortened from assisa, from assiso(“to make to sit, to seat, to place”)
etymonline
size (n.)
c. 1300, "an ordinance to fix the amount of a payment or tax," from Old French sise, shortened form of assise "session, assessment, regulation, manner," noun use of fem. past participle of asseoir "to cause to sit," from Latin assidere/adsidere "to sit beside" (and thus to assist in the office of a judge), "sit with in counsel or office," from ad "to" (see ad-) + sedere "to sit," from PIE root *sed- (1) "to sit."
Probably a misdivision of l'assise as la sise. The sense of "extent, amount, volume, magnitude" (c. 1300) is from the notion of regulating something by fixing the amount of it (weights, food portions, etc.). Specific sense of "set of dimensions of a manufactured article for sale" is attested from 1590s.
size (v.)
c. 1400, "to regulate," from size (n.). Meaning "to make of a certain size" is from c. 1600; that of "to classify according to size" is first attested 1630s. Verbal phrase size up "estimate, assess" is from 1847 and retains the root sense of size (n.). Related: Sized; sizing.
