I wanted to point the user to a FAQ but didn't find an appropriate one. I had the one on 'limitations on data' in mind, but that's been stripped and now merely says that 'You can find information on the data limitations of a feature in the associated FAQs'.
However, I don't know where the limitation on subject and entry length has gone to. The obvious one to me is the one on updating your journal, but there's nothing there about 100 characters/255 bytes for subjects and 65535 bytes or characters for entries (numbers which I got from the German version of that FAQ, which I found through a Google search, and which (fortunately, in this case) still has the old version.
Also, a FAQ search for '100' doesn't seem to find anything appropriate; one for '65535' returns nothing at all.
Suggestion: document the maximum length of subjects and comments in FAQ 222 on updating your journal.
Edit: I just found that FAQ 165 mentions the 65535 byte limit, but even that doesn't tell you how many bytes are in a subject -- and I still think the 65535 number would be good in the update journal FAQ as well.