If you are planning on setting a size here are the stats from w3schools.com
Higher – 17%
1024×768 – 57%
800×600 – 20%
640×480 – 0%
Unknown – 6%
The only problem I have with dynamic sizing i.e. width set to 100% is that you can not format your content precisely… so it looks different for each user.
Roo