Display all list thumbnail that already registered in WordPress.

thumbnail-list.php

<?php
function list_thumbnail_sizes(){
  	global $_wp_additional_image_sizes;
 	$sizes = array();
	foreach( get_intermediate_image_sizes() as $s ){
		$sizes[ $s ] = array( 0, 0 );
		if( in_array( $s, array( 'thumbnail', 'medium', 'large' ) ) ){
			$sizes[ $s ][0] = get_option( $s . '_size_w' );
			$sizes[ $s ][1] = get_option( $s . '_size_h' );
		}else{
			if( isset( $_wp_additional_image_sizes ) && isset( $_wp_additional_image_sizes[ $s ] ) )
				$sizes[ $s ] = array( $_wp_additional_image_sizes[ $s ]['width'], $_wp_additional_image_sizes[ $s ]['height'], );
		}
	}

	foreach( $sizes as $size => $atts ){
		echo $size . ' ' . implode( 'x', $atts ) . "\n";
	}
}
?>