Well is seem as though the magic numbers are 17312 Words, 98903 Characters. I did some testing and it seems to be more on the character count rather than the word count. As soon as it dropped to 98902 it would save. The actual charater
count with the html was 101625.
I cannot tell you why but that is what I found. The data type for this field in the DB is ntext and it should allow for this size. Are you using a DB or XML to store your post?
Maybe one of the developers will pipe in.