adding image sizes to wordpress

// Add new image sizes
if ( function_exists( 'add_image_size' ) ) {
    add_image_size('Product_Featured', 350, 150, TRUE);
    add_image_size('Product_Featured_Home', 275, 135, TRUE);
    add_image_size('Product_Featured_Ebook', 250, 400, TRUE);
    add_image_size('Product_Detail_Box', 200, 200, TRUE);
}

// Add custom image sizes to media dropdown choices
// ref link: http://ahjira.com/add-custom-image-sizes-to-media-library-size-selection-drop-down-list/
add_filter( 'image_size_names_choose', 'custom_image_sizes_choose' );
function custom_image_sizes_choose( $sizes ) {
    $custom_sizes = array(
        'Product_Featured' => 'Product_Featured',
        'Product_Featured_Home' => 'Product_Featured_Home',
        'Product_Featured_Ebook' => 'Product_Featured_Ebook',
        'Product_Detail_Box' => 'Product_Detail_Box',
    );
    return array_merge( $sizes, $custom_sizes );
}