In the "FORUMHOME" group templates you'll find these two templates:
forumhome_forumbit_level2_post
forumhome_forumbit_level1_post
Where you see:
Code:
<td class="alt2" nowrap="nowrap">$forum[lastpostinfo]</td>
Add the width:
Code:
<td class="alt2" width="200px" nowrap="nowrap">$forum[lastpostinfo]</td>
Change "200px" to whatever you want.
I find that <div> tags within the <td> tags with width settings are more accurate. For an edit such as this, however, I think the above code will work fine.