Adding CSS to Blog Posts?

You can use PHP code in your child themes functions.php file to add a custom body class to any page/post etc.

add_filter( 'body_class', 'my_custom_body_class' );
function my_custom_body_class( $classes ) {
if ( is_single( '007' ))
    $classes[] = 'custom-class';
    return $classes;
}

This sample code will add a custom body class to the post with i.d of 007 so you can style that specific post in your child themes style.css file.

Sample CSS:

.custom-class {
font-size: 20px;
}

Source http://codex.wordpress.org/Function_Reference/body_class