". But it had an influence in placing obstacles in the way of her
association with Mrs Gowan by making the Prunes and Prism school
excessively polite to her, but not very intimate with her; and
Little Dorrit, as an enforced sizar of that college, was obliged to
submit herself humbly to its ordinances.

a sizar (a student who received money for college expenses in exchange for being a servant to other students).

Heavens! And what of the etymology of this very strange-looking word?

"Etymology: sizar alteration of sizer, from 1size
Date: 1588"

MW

Now that 1size, according to my online search, comes from assize, which makes no sense at all:

"1 a : a judicial inquest b : an action to be decided by such an inquest, the writ for instituting it, or the verdict or finding rendered by the jury"