You may want to try using the max-width CSS property, however this is ignored by IE6- (for modern browsers it's fine). You can see it implemented in RuneWire, where the content expands with the page/window until a certain point and then it stays at that size. In essence it will allow you to set well.. a maximum width! Which works well in conjunction with a percentage width because you can set the maximum to use a different, perhaps more precise measurement such as pixels.
With a table, you'll probably want something like:
<table width="x%" height="x%" style="max-width: 1280px">
[table structure]
</table>
Hopefully this is what you're looking for.
