Create a custom size for your new image uploads. Add to functions.php

Custom size images in WordPress

// custom image size for slider

if ( function_exists( 'add_image_size' ) ) {
  add_image_size( 'new-size', 900, 400, true ); //(add new size here)
}

add_filter('image_size_names_choose', 'my_image_sizes');
function my_image_sizes($sizes) {
        $addsizes = array(
                "new-size" => __( "New Size")
                );
        $newsizes = array_merge($sizes, $addsizes);
        return $newsizes;
}