Get all the registered image sizes along with their dimensions

wp-get-image-sizes.php

<?php
/**
 * Get all the registered image sizes along with their dimensions
 *
 * @global array $_wp_additional_image_sizes
 *
 * @link http://core.trac.wordpress.org/ticket/18947 Reference ticket
 * @return array $image_sizes The image sizes
 */
function _get_all_image_sizes() {
	global $_wp_additional_image_sizes;

	$default_image_sizes = array( 'thumbnail', 'medium', 'large' );
	 
	foreach ( $default_image_sizes as $size ) {
		$image_sizes[$size]['width']	= intval( get_option( "{$size}_size_w") );
		$image_sizes[$size]['height'] = intval( get_option( "{$size}_size_h") );
		$image_sizes[$size]['crop']	= get_option( "{$size}_crop" ) ? get_option( "{$size}_crop" ) : false;
	}
	
	if ( isset( $_wp_additional_image_sizes ) && count( $_wp_additional_image_sizes ) )
		$image_sizes = array_merge( $image_sizes, $_wp_additional_image_sizes );
		
	return $image_sizes;
}
?>

@mikedamoiseau

Put global $_wp_additional_image_sizes at the beginning.