The idea is not to put more content in the table than can fit. What do you want to happen to the content that extends beyond the boundary? Anyways, the css property you would be dealing with is overflow. Tables do not respond real well to overflow. You can however, nest the table in a division:
The above table will never appear wider than 300px. Anything in it that lies beyond its 300px x-offset will be invisible. You can use 'overflow:scroll' then a scrollbar will appear to allow viewing of the clipped material (if any).