How apply width to a column in a datatable?

Hi. I would like to know how can i apply the property width to a column in a datatable?. The problem is i have a column with some text, like "text1 my text" in the header and just one word in the column. I want to reduce the width to the minimum using demo-table.css but its not working. Any advice?

For example:

<thead>
<tr>
<th class="enca1">text1 my text</th>
</tr>
</thead>
<tbody>
<tr class="odd gradeX">
<td class="columna1">text</td>
</tr>
</tbody>

Thanks!!