I’d like to tip you of Juicy Studio’s styleswitcher: http://juicystudio.com/index.php – click on the Switch to High Contrast-link in the menu. This also works without any scripts, as it’s using PHP with cookies (I presume) to check for user-style-setting.
You could use something like that where the user could click to get different text-sizes (through different stylesheets). A List Apart also has an old article about creating your own style switcher.
