WordPress | Get a list of all the possible thumbnail sizes set within a theme

This time i need to work on a customer website developed by a colleague, i don’t know witch images size he had registered.
So, i need print a list of all images sizes.

Addi this code whet ever you want the list will be print.

print_r(get_intermediate_image_sizes());

and this is the result:

If you need Name and sizes :

Add this in your function file:

/**
 * Get size information for all currently-registered image sizes.
 *
 * @global $_wp_additional_image_sizes
 * @uses   get_intermediate_image_sizes()
 * @return array $sizes Data for all currently-registered image sizes.
 */
function get_image_sizes() {
	global $_wp_additional_image_sizes;

	$sizes = array();

	foreach ( get_intermediate_image_sizes() as $_size ) {
		if ( in_array( $_size, array('thumbnail', 'medium', 'medium_large', 'large') ) ) {
			$sizes[ $_size ]['width']  = get_option( "{$_size}_size_w" );
			$sizes[ $_size ]['height'] = get_option( "{$_size}_size_h" );
			$sizes[ $_size ]['crop']   = (bool) get_option( "{$_size}_crop" );
		} elseif ( isset( $_wp_additional_image_sizes[ $_size ] ) ) {
			$sizes[ $_size ] = array(
				'width'  => $_wp_additional_image_sizes[ $_size ]['width'],
				'height' => $_wp_additional_image_sizes[ $_size ]['height'],
				'crop'   => $_wp_additional_image_sizes[ $_size ]['crop'],
			);
		}
	}

	return $sizes;
}

/**
 * Get size information for a specific image size.
 *
 * @uses   get_image_sizes()
 * @param  string $size The image size for which to retrieve data.
 * @return bool|array $size Size data about an image size or false if the size doesn't exist.
 */
function get_image_size( $size ) {
	$sizes = get_image_sizes();

	if ( isset( $sizes[ $size ] ) ) {
		return $sizes[ $size ];
	}

	return false;
}

/**
 * Get the width of a specific image size.
 *
 * @uses   get_image_size()
 * @param  string $size The image size for which to retrieve data.
 * @return bool|string $size Width of an image size or false if the size doesn't exist.
 */
function get_image_width( $size ) {
	if ( ! $size = get_image_size( $size ) ) {
		return false;
	}

	if ( isset( $size['width'] ) ) {
		return $size['width'];
	}

	return false;
}

/**
 * Get the height of a specific image size.
 *
 * @uses   get_image_size()
 * @param  string $size The image size for which to retrieve data.
 * @return bool|string $size Height of an image size or false if the size doesn't exist.
 */
function get_image_height( $size ) {
	if ( ! $size = get_image_size( $size ) ) {
		return false;
	}

	if ( isset( $size['height'] ) ) {
		return $size['height'];
	}

	return false;
}

and then, where you want to print the array :

var_dump( get_image_sizes() );

and this is the result: