E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The quota of food allowed at breakfast, and also food sized for at dinner. At Cambridge, the students are allowed meat for dinner, but tart, jelly, ale, etc., are obtained only by paying extra. These articles are called sizings, and those who demand them size for them. The word is a contraction of assize, a statute to regulate the size or weight of articles sold. (See SICE.)
A size is a portion of bread or drinke: it is a farthing which schollers in Cambridge have at the buttery. It is noted with the letter S.Minshen. (See also Ellis: Literary Letters, p. 178.)