Something like this:
code:
<!--THIS PART GOES INTO THE GENERIC HEADER-->
<div align="center">
<table width="770" border="0" cellspacing="0" cellpadding="0">
<tr>
<td align="center" valign="top">
THREADS WILL END UP GOING IN HERE
<!--THIS PART GOES INTO THE GENERIC FOOTER-->
</td>
</tr>
</table>
<p></p>
</div>
Then where it says 770 (which is a fixed pixel width) you can change to 100%.
So when you want to see what your site looks like on a very small monitor... change the width from 100% to to a fixed width like 640 etc....
Normally I would leave it on 100% so the site will expand to the full width of everyone's monitor.
Hope that helps.