In photoshop, layout size pixel is important for resolution.
For web pages though it is just about completely irrelevant. The only things that should be measured in pixels on the web are images and minimum width borders. All other sizes should be measured in % or em.
When you use % for sizes they will be set relative to the browser viewport size or the container they are inside. The maximum width of individual columns for readability should be around 30 to 35em.
Felgall touches on some important points. When you develop pages, you will want to use measurements that are relative and that are easily interpreted for optimal display quality by the browser. When you use absolute values, then your pages will not render correctly on most machines.
Here is an example article on font size to give you an idea,