You can use a percentage on your <div>s. Try:
Setting 'min-height' is not the same as setting 'height'. Without setting 'height' on #wrap, your internal <div>s don't have a reference for setting their own height as a percentage. I also changed #bottom to 20% so that the total of the two was 100%. While you can use 30%, it makes the document exceed the size of the browser window and I'm not sure that's what you wanted.