get dimensions of an image size

get_image_size.php

<?php
// use the $_wp_additional_image_sizes global to fetch the dimensions of a given image size.
// this comes via Otto, who is amazing and knows everything: http://wordpress.stackexchange.com/a/28316

function get_image_size( $size = 'thumbnail' ) {
	global $_wp_additional_image_sizes;

	$width = $_wp_additional_image_sizes[$size]['width'];
	$height = $_wp_additional_image_sizes[$size]['height'];

	return array( 'width' => $width, 'height' => $height );
}