You need to change the size of the field in the database and change the limit in PHP. More info on how to do this in this question: http://www.question2answer.org/qa/8870
A must read related question is here: http://question2answer.org/qa/39622
Now, I think I've mentioned this in some other answer but it case it was just my imagination I'll say it again. Changing the value in is risky. If you are not planning to upgrade Q2A then you won't have any issue with it. However, if you plan to upgrade it when new versions are released then you'll have to check that this field is not modified to a lower value than the one you set.
For example, suppose you change the field value to 10k. Then you upgrade Q2A and the new version sets the value to 9k. This means that all posts that have over 9k bytes will be truncated to 9k during the upgrade. Definitely not good. You could perform a check before doing each upgrade and manually hack the installation process so that the field does not get modified... but there is always the chance to mess with that (we're humans after all!).
Also bear in mind that the 8000 value is tricky. It is not measured in characters but rather bytes. Some characters could take 2 bytes to be represented. Also note that 8000 bytes also hold format information (eg: bold) which, depending on the editor, might add more or many more bytes that take additional space.