sizar (n.)

c. 1300, "quantity, length, stature; manner, method, custom; a decision, a stipulated reward," 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." The French word is probably a misdivision of l'assise as la sise.

The sense of "extent, amount, volume, magnitude" is from the notion of regulation (of weights, food portions, etc.) by fixing the amount of it. The specific sense of "one of a set of regularly increasing dimensions of a manufactured article for sale" is attested from 1590s (in reference to shoes). Figurative use of the sales clerk's try (something) on for size (to see if it "fits") is by 1956.