字体大小相对于用户的屏幕分辨率？

I have a fluid website and the menu is 20% of its width. I want the font size of the menu to be measured properly so it always fits the width of the box and never wrap to the next line. I was thinking of using "em" as a unit but it is relative to the browser's font size, so when I change resolutions the font size stays the same. I have a fluid website and the menu is 20% of i