Setting to percentage instead of pixel is difficult because of the header image. Not impossible, it will just distort your header image depending on how wide your monitor is.
I really wouldn’t recommend setting to a percentage instead of a pixel width because some folks have very wide monitors and setting your blog to a percentage would make it very difficult to read.
That said, there have been a lot of discussions on this forum in the past month or so about changing the width of things. Do a search around in here and I’m sure you’ll find it.
Ken