i would suggest that you float your tables on 100%. the main advantage on using percentages is that you would style the sheet according to most people's screens. visit sitepoint.com for more info on styling on CSS. they have a huge selection on books for you to sort your problem.
Actually <div> element is 100% stretched by default, unless contained in another block element with fixed widht, or unless floated. So, if you are using div's and do not use neither float, nor width properties, they will be still 100% wide. Now, what is your goal so we could help more?