There's really no need to set a height on the footer. In general, setting heights on anything is a bad idea, because it can lead to text spilling out of boxes etc, so I never set heights.
If you are worried about the footer ending up too high on some pages with little content, a better solution would be to set a min-height on the content area, so that the footer is always at least a certain distance down the page.
E.g. If you set the .content area to min-height: 600px; then at least there will always be some space on the page before the footer.