How to display custom style based on theme option select box

Just put your dynamic CSS inside of a callback function, hooked into wp_print_script, e.g.

function mytheme_custom_css() {
    ?>
    <script type="text/css">
    <?php if ( of_get_option('show_post_sidebar', true ) ) { ?>
        .post{width:720px}
    <?php } ?>
    </script>
    <?php
}
add_action( 'wp_print_scripts', 'mytheme_custom_css' );

Add as few, or as many, conditional CSS rules as you need.