I found this:
<textarea name="message" id="message" rows="2" cols="40" class="qa-form-tall-text">
EDIT: The code above is the size of the text area. Nothing to do with the limit. Apart from that I've Cmd + F 'd every part of the text area for 'Maxlength' and 'input' and no results. So I'm not sure there's one. Although PM could of hid the code which is possible but I don't see a reason for it. What I was looking for is this basically:
<input id="message" class="qa-chat-post input-text" type="text" name="ajax_add_message" autocomplete="off" maxlength="800">
This came from chat and If you look it says:
This means the maximum character limit for a chat post is 800 words. Because I can't find that on the wall script that's what leads me to believing there's no Character limit on walls.