An important consideration for user experience is to note the max width/height of any particular area that content will reside. Always design according to the maximum amount of content an area should hold. If you dont, then you will have issues when longer blocks of text are added to certain content areas of your application. You should also consider background images (repeating, scaling, % based, etc) and how one div height might affect another (wrapping, floats and column based layouts).

Always test your HTML with lots of content to see how it reacts. You can save yourself a lot of trouble later.