My advice is to set your font-family in the BODY part of your css, then set the font-size: to between 70 and 80% (depending on font)

Then in the other styles use EM font sizes based on 1 (1 being the same size as the 70%-80% of the font)

Example:

BODY {
font-family: Arial, Helvetica, sans-serif;
font-size: 76%;
color: #000;
}

H1 { 1.4em } /* header 1 is 140% size of standard text */
H2 { 1.25em } /* header 2 is 125% size of standard text */
H3 { 1.1em } /* header 2 is 110% size of standard text */

As you build your page, resize the text to check that it doesn't break the layout.