This Wikipedia gadget adds "Article stats" to the top menu and shows some size-related data:

• total article size in bytes,
• word count (excluding tables, quotes, lists, titles, footnotes and comments),
• character count (excluding tables, quotes, lists, titles, footnotes and comments).

This gadget works for the currently open revision as well as the diff page.

Buidling

gulp

builds and minifies the script to ./dist/wiki-article-size.js

gulp --usercript /path/to/firefox/profile/

builds the script as a userscript to /path/to/firefox/profile/gm_scripts/wiki-article-size/wiki-article-size.user.js.

You can also use

gulp watch

with or without --userscript to automatically build the script whenever any of the source files change.