Create 3 different style sheets:
pc.css
pcbig.css
pchuge.css

Use the following code for the style sheet:
<link rel="stylesheet" type="text/css" href="pc.css" title="small" />
<link rel="alternate stylesheet" type="text/css" href="pcbig.css" title="med" />
<link rel="alternate stylesheet" type="text/css" href="pchuge.css" title="large" />

Use the following code for the links:
<b> Text Size:</b> <a href="#" onclick="setActiveStyleSheet('small'); return false;">Sm</a> | <a href="#" onclick="setActiveStyleSheet('med'); return false;">Med</a> | <a href="#" onclick="setActiveStyleSheet('large'); return false;">Lg</a>