You can also use percentages, and/or specify in the column headers:
<table width="300">
<tr>
<th width="20%">Column 1</th>
<th width="20%">Column 2</th>
<th width="20%">Column 3</th>
<th width="20%">Column 4</th>
<th width="20%">Column 5</th>
</tr>
<tr>
<!--- row data -->
</tr>
</table>
The bonus with percentages is lower code maintenance: you can change your table width without having to re-specify the column widths.
Caveat: It is my understanding that table width specified in pixels isn't supported in HTML 5; you need to use CSS instead.