I see many websites that use width at 100% so as to cater to everyone's monitor resolution. That may sound great but you have to understand that not all websites can look good at 640 or at 1050. You have to pick and choose, and I don't suggest you choose 100%.
I use to use 100% but I noticed that my heading never usually look good on a larger screen or always seemed to be cut off on the smaller screen, which really defeats the purpose of it.
what I suggest is to pick a resolution bewtween 640 and 800 so as to fit on any large screen and to not force small screens to scroll too much to the left of right.
I myself favour using 720 because it just about fits on the small screen and will fit on any large screen. Also, using pixel as oppose to percentage makes your website look the same on any computer. The same can't be said about percentages.
The larger your width, the more content you have to fill. This isn't a very good idea if you are a one man show. Can't just put up any content for the sake of filling space.
Hart


