That question bugs me to try and find the answer for every time, so this time I'm going to put it in a lot more wiki places, ha. Anyway, the answer seems to be 300,000 characters including spaces, or 450k bytes

I do not mind questions, although I note that commenting on dw_news is probably more effective! Right now I think the crossposter just fails and gives a notification of failure, but I think at some point they want to post just a link to the longer post. (Automatically truncating the post runs into problems: what if you cut in the middle of an HTML tag?) I also received reports that when you edit the post and make it shorter, that works.