In your FORUMHOME template, look for the following code:
Code:
<tr align="center">
<td class="thead"> </td>
<td class="thead" width="100%" align="$stylevar[left]">$vbphrase[forum]</td>
<td class="thead" width="175">$vbphrase[last_post]</td>
<td class="thead">$vbphrase[threads]</td>
<td class="thead">$vbphrase[posts]</td>
<if condition="$vboptions['showmoderatorcolumn']">
<td class="thead">$vbphrase[moderator]</td>
</if>
</tr>
you can change the existing "width" tags to suit your needs, and / or add additional width tags to $vbphrase[threads] and $vbphrase[posts], right after "thead" like the previous lines.
If you have a custom style, this code might be located in your forumhome_forumbit_level1_nopost template.