Using CSS you can use absolute units, but remember that if someone has a visual disability, it's not really fair to stop them resizing their text so they can actually read it:
From the CSS 2 spec:
in: inches -- 1 inch is equal to 2.54 centimeters.
pt: points -- the points used by CSS2 are equal to 1/72th of an inch.
pc: picas -- 1 pica is equal to 12 points.
You are only really supposed to use these on Physical mediums, such as paper (e.g. when printed) though. And also in some browsers (IE I think) px doesn't resize as it's supposed to.