Index: wp-admin/includes/media.php
===================================================================
--- wp-admin/includes/media.php	(revision 18607)
+++ wp-admin/includes/media.php	(working copy)
@@ -868,6 +868,8 @@
 
 		// get a list of the actual pixel dimensions of each possible intermediate version of this image
 		$size_names = array('thumbnail' => __('Thumbnail'), 'medium' => __('Medium'), 'large' => __('Large'), 'full' => __('Full Size'));
+		// allow plugins/themes to add their custom defined image sizes
+		$size_names = apply_filters('image_size_input_fields_sizes', $size_names);
 
 		if ( empty($check) )
 			$check = get_user_setting('imgsize', 'medium');