See: http://www.html5-tutorials.org/tables/changing-column-width/
After the table tag, use the col element. you don't need a closing tag.
For example, if you had three columns:
<table>
<colgroup>
<col style="width:40%">
<col style="width:30%">
<col style="width:30%">
</colgroup>
<tbody>
...
</tbody>
</table>