// Add new image sizes:
add_image_size( 'thumb-512x384', 512, 384, true );
add_image_size( 'thumb-640x480', 640, 480, true );
// Register the sizes:
add_filter( 'image_size_names_choose', 'my_custom_image_sizes' );
function my_custom_image_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'thumb-512x384' => __( 'Thumbnail 512x384px' ),
        'thumb-640x480' => __( 'Thumbnail 640x480px' ),
    ) );
}