I’ve run across this before. The usual way to limit input to a text field is to have the limits set in the field itself using the maxlength attribute. That means hacking the TV code. The code is on line 25 of the file manager/includes/tmplvars.inc.php:
$field_html .= '<input type="text" id="tv'.$field_name.'" name="tv'.$field_name.'" value="'.htmlspecialchars($field_value).'" '.$field_style.' tvtype="'.$field_type.'" onchange="documentDirty=true;" style="width:100%" />';
http://htmlhelp.com/reference/html40/forms/input.html
That doesn’t help with texareas (line 34 of the same file), however.
Here’s a good discussion on the topic (there is a typo where he says input type="file" where he means type="text")
http://www.cs.tut.fi/~jkorpela/forms/textarea.html
Another interesting bit of javascript to actually stop the user from entering more than the desired number of characters (although this can be bypassed if javascript is turned off, but then the Manager wouldn’t work at all):
http://www.mediacollege.com/internet/javascript/form/limit-characters.html