I've made some changes to try and make it more explicit that any method of adding formatting (not just adding HTML tags in the HTML editor) will increase the number of characters in the entry. I also expanded the title, made a note about how many characters the limit corresponds to, and split the first para into two, thus:
Why can't I update with long entries? What is the maximum entry size?
Entries are limited to 65,535 bytes in size. The exact number of characters this corresponds to will vary depending on the language you are typing your entries in. If you wish to post entries larger than the limit, you will need to post multiple entries instead.
If you have added formatting to your entry by using HTML tags, the Rich Text editor, a third party client or a text editing application, this will add additional characters that will count towards the limit. You may be able to reduce the size of your entry by switching to the HTML editor, removing unneeded formatting tags, and trying to post the entry again.
(I didn't touch the third para)
I'd also like to give users some sort of idea of how many characters the 65,535 byte limit corresponds to. This is reasonably straightforward in Latin character sets, as it's close enough to one-to-one, but it gets complicated when we talk about, say, Cyrillic or Asian character sets. I thought of replacing the second sentence of para 1 with something like
In most Latin character sets, this is about 65,000 characters. In Asian and Cyrillic character sets, this is about 33,000 characters.but I'm not sure that that second value is correct. Any suggestions?