Fixed Table Cell Width

You could try using the <col> tag manage table styling for all rows but you will need to set the table-layout:fixed style on the <table> or the tables css class and set the overflow style for the cells

http://www.w3schools.com/TAGS/tag_col.asp

<table class="fixed">
    <col width="20px" />
    <col width="30px" />
    <col width="40px" />
    <tr>
        <td>text</td>
        <td>text</td>
        <td>text</td>
    </tr>
</table>

and this be your CSS

table.fixed { table-layout:fixed; }
table.fixed td { overflow: hidden; }

Now in HTML5/CSS3 we have better solution for the problem. In my opinion this purely CSS solution is recommended:

table.fixed {table-layout:fixed; width:90px;}/*Setting the table width is important!*/
table.fixed td {overflow:hidden;}/*Hide text outside the cell.*/
table.fixed td:nth-of-type(1) {width:20px;}/*Setting the width of column 1.*/
table.fixed td:nth-of-type(2) {width:30px;}/*Setting the width of column 2.*/
table.fixed td:nth-of-type(3) {width:40px;}/*Setting the width of column 3.*/

<table class="fixed">
    <tr>
        <td>Veryverylongtext</td>
        <td>Actuallythistextismuchlongeeeeeer</td>
        <td>We should use spaces tooooooooooooo</td>
    </tr>
</table>

You need to set the table's width even in haunter's solution. Otherwise it doesn't work.
Also a new CSS3 feature that vsync suggested is: word-break:break-all;. This will break the words without spaces in them to multiple lines too. Just modify the code like this:

table.fixed { table-layout:fixed; width:90px; word-break:break-all;}

Final result