// Add additional image sizes
add_image_size( 'homepage-thumb', 535, 350, true );
// Show new image size in admin are of media section
add_filter( 'image_size_names_choose', 'yourTheme_custom_image_size' );
function yourTheme_custom_image_size( $sizes ) {
return array_merge( $sizes, array(
'homepage-thumb' => __( 'Homepage Thumbnail' ),
) );
}