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.
# cm: centimeters
# mm: millimeters
# 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.
The problem is that operating systems work with pixels and browsers do not know how many pixels there are to an inch on a user's screen. Always using pixels will encourage your page to keep its proportions; but as Rick pointed out, it's not nice to do that to those of us with failing eyes.
Alright, this is the problem I have. I have to columns of a table, next to each other, each column with its own background image, and the two images line up. I have specified the width of the left column to 113 pixels. Now, when you view the page with larger font ir looks really crappy. Go check yourselves: www.uwasa.fi/~h79188/artikkelit.html . Do you have any suggestions on how to fix that without locking the font size?
My very petite contribution is that your problem is not the font size, but that you're using tables to layot the page... big no-no!! For the layout you chose (inverted L) I'd make the whole image your background and use images instead of text for the menu items. This way you keep proportionality and size of the text in full control. The downside is that download time may go up if you're not smart about the way you make the graphics. Go transparent GIF and try to keep the graphics to less that 1 KB each (it's doable). Don't forget the ALT text!