how to change # of tag posts on /tag page?

I would put this in functions.php:

function main_query_mods( $query ) {
    // check http://codex.wordpress.org/Conditional_Tags to play with other queries
    if(!$query->is_main_query()) {
        return;
    }
    if(is_tag()) {
        $query->set('posts_per_page',15);
    }
}
add_action( 'pre_get_posts', 'main_query_mods' );