How to disable fancybox on small screens

Question

You could have a try with wp_is_mobile():

if ( wp_is_mobile() ) {
  remove_action( 'wp_print_scripts', 'mfbfw_load' );
  remove_action( 'wp_print_styles', 'mfbfw_css' );
  remove_action( 'wp_head', 'mfbfw_init' );
}

You should also check with tablets since they might have a big resolution and hence not fall in your “small screen” condition. You should note that the User Agent string can be modified by any browser sending the request.