Assuming you are referring to thread titles, I believe all you have to do is modify this: Last Thread Title Maximum Displayed Characters
vBulletin Options - Forum Listing Display Options - Last Thread Title Maximum Displayed Characters
eta: Just ran across this... maybe this is what you're actually looking for...
Edit this line in the
newthread template:
Code:
<td><input type="text" class="bginput" name="subject" value="$subject" size="40" maxlength="85" tabindex="1" /></td>