Decompress and upload the contents of the archive into /wp-content/plugins/.
Activate the plugin on your WP Admin > Plugins page by clicking 'Activate' at the end of the 'WP-chgFontSize' row.
Configure the plugin on your WP Admin > Options > Font Size page.
To use it, there are two possibilities:
If your theme supports widgets, and you have installed the widget plugin or you are using Wordpress 2.2 or higher, add the 'WP-chgFontSize' widget on your WP Admin > Presentation > Widgets page.
Add <?php chgfontsize_display_options(); ?> at the place in the theme's file where you want the font size selection appear.