Fixed ...
Readability = Usability
"We" are accustom to reading in a defined width/height ... eg. books, letters, etc.
- People don't like reading more than x amount of words per line ... newspapers are broken up into columns for a reason
- Everyone sees the same thing that you see. eg. equal amount of text; equal amount of white space, etc
TIP- You should always design with the lowest common denominator in mind.
- A fixed layout is not always best. For example, a site primarily to showcase photos would be best in a variable (liquid) width layout.
- If using a variable layout ... make sure the text area is a fixed width this will help ensure that your readers will stick around to read the entire text.