One of my colleagues just pointed this out to me:
Almost everywhere else in Moodle where students can upload files, there is a way for the teacher to set a maximum allowed file size. This is not possible for Essay questions. (As far as I can see from checking the code, it just uses the site-wide file upload limit - $CFG->maxbytes.)
Presumably it would be sensible to add an option on the form for creating essay question?