I poked various people and asked if this was covered in the FAQs somewhere, and while there is a section in FAQ 210 addressing sudden font-size changes, it doesn't mention the possibility that the stylesheet failed to load (and that often, forcing a reload will fix the problem). I know it's mentioned later on in a separate section, but I don't think it'd hurt to address it as a possibility within the "Font Size" section itself. Something along the lines of:
"It is also possible that LiveJournal's style sheet, which controls some aspects of font sizing for users using the XColibur site scheme, may have failed to load properly. Refreshing the webpage often solves this problem."
Might also need a link to FAQ 160 to deal with the possibility of them viewing cached webpages?