Make WordPress generate additional custom-sized images

Add this to your function.php file and change the highlighted sizes and labels to what you’d like. Visit the WordPress Codex for more information on the add_image_size() function:

// Additional Image size

add_image_size('widescreen', 750, 425, false);

add_filter('image_size_names_choose','your_custom_image_sizes');

function your_custom_image_sizes($sizes) {
    return array_merge($sizes, array('widescreen' => 'Widescreen'));
}