It's a common default on asp & php forums with very long posts - there's a memory or word count 'limit' to what you can upload and the only remedy is to split the post. I get it a lot on my other forum & that's also when I put up a goodly amount of fanfic mostly.
The other one's an asp board so it may be that's slightly different on here and it also depends on what you're posting as well as I've been slapping on some tables and graphics as well recently and those take up more 'space' too. With straight text on a 1024 wide pixel screen I find the limit's about 10-12 scroll clicks (below the slider and not on the tab) in the post window, which comes out as 6-8 scroll clicks on the actual browser window. As a rule of thumb if your post window scroll slider is 0.3 cm or less then you're reaching the limit and need to start looking for a good place to break it up.
Another option if you're typing the post in Word or whatever then I'm finding that roughly 3 sides of A4 will generally fit into one post window OK. Typing in Arial 9