Not sure that limiting the textarea would help - it's not like it constrains the length of these replies, for example. It's pretty trivial to compose a block of text in Textmate and paste it in, I'll often do that just for the spell checking or to make sure it doesn't get lost in submission or browser crash or whatever. (and often to review and make sure I don't post something I'll regret)

I believe that this form submits to a process on a server that could limit the number of characters that are allowed in the textarea.