table-layout: fixed
will get force the cells to fit the table (and not the other way around), e.g.:
<table style="border: 1px solid black; width: 100%; word-wrap:break-word;
table-layout: fixed;">
<tr>
<td>
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
</td>
</tr>
</table>