Does it make sense to optimize for different screen sizes by using a percentage (floating) of the screen or a fixed size for the screen width.
Considering the fact that browsers are windows and may take up any portion of the screen the end user wishes, plus the fact that most browsers come with diferent toolbars, sidebars and icon size options, choosing a fixed size seems a bit of a fool's errand to me. It's an errand plenty of our customers ask us to run, so ultimately the customer gets what the customer wants, but if you're talking about developing your own personal production standards, I think it makes great sense to disregard fixed sizes of any kind.
it's the cross browser compatibility comes into picture... it's good if you use position:fixed for no problems in any browser.
I try to design so my sites look similar to users of 800x600 to about 1400x whatever... Fixing makes you either scroll sideways, or gives a big big margin on both sides, which kind of doesn't look good... Bye
Fixed width is easier and causes you less problems. But it really depends on your visitors and what kind of site you want to have for them.