hi there,
you could have a close look at forumlist_body.html and viewforum_body.html, seach for:
<th> {L_LAST_POST} </th>
3 hits in total i believe
but i don't know what would be wise to do there, guess thats up to you and what you want to do
hope this helped alittle
btw usually it will automatically grow bigger when the content is larger