This script adds a Page size link to the toolbox, i.e. the box in the left hand column (by default) which also contains What links here (among other things). Clicking on this link displays some statistics about the page and prose size (see below), and highlights the 'readable prose'. Clicking the link again turns these off.
NB: This script is identical to User:Dr pda/prosesize.js, with the exception that all sizes are shown in bytes, not kilobytes.