It is better to use a fixed size. While making the site size fits nicely to every resolution sounds nice, it takes lost of effort.
First you have to start with the design, you must plan ahead how the site would look like in different resolution.
Your current site, for example, would look really weird if you stretch the navigation bar any wider. However, you can have the "black" background of the navigation bar stretch 100% across the page if you want. You will have to make changes on your current design to get that though. Right now your "main_w" is restricting the max width of the elements.