Around the time I wrote a language compiler using a richtext box, I do seem to recall that the amount of text was basically unlimited. Anyway what you could do is hook into the KeyDown method and implement a character counting routine. It gets tricky because you have to track when the back arrow is used to insert a character, when the delete key is hit, when the backspace key is hit, etc. But for testing purposes (and other usability reasons) you could include a small text box that displays the character count on the side. Also add a tooltip that states the max character limit. Put everything in place and then start to pound the daylights out of it and ensure the character count comes out right. You should, however, be very aware of the fact that the character count will not define the length of text in the result. The reason is because everything is wrapped in Rich Text format characters which will add extra length to everything no matter what you do.
