Hmm... it worked without a hitch when I tried it on my own test forum, and changing the maxlength value for other input fields in the templates has always worked before as well, so besides going through
Admin > Themes and Layout > Modify Themes > Browse the templates and files in this theme. to make 100 per cent sure that the right template has been edited, I'm out of ideas

Maybe you could try to decrease the value below 30 first, to see if that has any effect?