With the CSS font-size element, you should apply the size as an 'em' so that when the end-user changes the text size in their browser (View->Text Size->Small/Medium/Large etc), the font can resize.
This can sometimes cause issues with your design/layout if a user has their text size set to Largest, but it's more of an accessibility feature...making your site/application available to those who can read normal text, and to those who need larger text.
font-size:1em;
1 is the 'standard' size, but you can increase/decrease it to suit what you need (in increments of 0.1) 0.8, 0.9, 1, 1.1, 1.2 and so on