Add custom image sizes in functions.php After that regenerate Thumbnails with regenerate thumbnails plugin.
<?php
if (function_exists('add_theme_support'))
{
// Add Thumbnail Theme Support
add_theme_support('post-thumbnails');
// Add sizes
add_image_size('lightbox', 2000, '', true);
}
// Add sizes to backend
function my_custom_sizes( $sizes ) {
return array_merge( $sizes, array(
'lightbox' => __( 'Lightbox' ),
) );
}
add_filter( 'image_size_names_choose', 'my_custom_sizes' );
?>
thumbnail // Thumbnail (default 150px x 150px max)
medium // Medium resolution (default 300px x 300px max)
large // Large resolution (default 640px x 640px max)
full // Original image resolution (unmodified)