Which Font-Size? Px, Pt or Em
Using CSS (cascading style sheets) makes it easy to specify font sizes, but before you set a font size you should be aware that it could change the layout of your site considerably. Different browsers interpret font sizes differently, so a font that appears readable in Microsoft Internet Explorer may be smaller when viewed in Netscape. In addition, font sizes on Windows systems are not always the same as they are on other platforms. Your site may look great to Windows users, but it may be illegible to those using a Mac.
When you specify a font size, specify it in pixels (px) not points (pt) or em. Using a pt or em font-size property instead of px allows for your site text to be resized according to the viewer's system settings. If their system is set to view very large text, your web site's layout will become distorted and your web site may be illegible to them.
Also, be very careful not to set your font-size pixels too small. Some folks may not be able to read tiny text and adjusting their system text size will have no effect on your site because your font-size is specified as px. There truly is a happy medium in any situation and the font-size (ie. 12px) will vary depending on the font-family (ie. Arial, Times New Roman, etc.) you use.