Sounds a bit like your doing something odd?
If you dont want the id: sidebar to scale then set a height/width in the css and overflow: hidden. That should stop the class sidebar pushing the id sidebar down/accross.
Although if your just adding a background image you could just apply it to sidebar id? Also you have set the sidebar class height and width but not the sidebar id height?
If you have a jpg of what your trying to do that would be ideal as I think you might be making things harder on yourself with your html / css!