Messages have a size limit to accomodate speed and indexing strength.
Analyzing the boards, most of the extremely large posts are actually copied texts of information. So thus the limit seemed a well placed thing.
If you wish to post a large message such as that, cut and paste half of it into a reply of the topic. (ie write the post, cut the bottom half (control+x) then go to the post, hit reply and paste (control+v) the other half in.)
I have seen very FEW posts which are hand typed messages which exceed 4000 characters and those that do usually do so by maybe 100 characters.