Another option would be to modify the
account_personal_details template.
Find the line:
<input type="text" name="custom_title" value="{$visitor.custom_title}" id="ctrl_custom_title" class="textCtrl" />
and change to:
<input type="text" name="custom_title" value="{$visitor.custom_title}" id="ctrl_custom_title" class="textCtrl" maxlength="18"/>
Then you're just preventing someone from entering more than 18 characters and you don't have to worry about any of the code changes being overwritten when you install your next upgrade. Hard core programmers might know how to get past this but your average user community probably will not.
--Ed