Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 49020)
+++ src/wp-includes/media.php	(working copy)
@@ -379,7 +379,8 @@
 
 	$title = $title ? 'title="' . esc_attr( $title ) . '" ' : '';
 
-	$class = 'align' . esc_attr( $align ) . ' size-' . esc_attr( $size ) . ' wp-image-' . $id;
+	$size_class = is_array( $size ) ? implode( 'x', $size ) : $size;
+	$class      = 'align' . esc_attr( $align ) . ' size-' . esc_attr( $size_class ) . ' wp-image-' . $id;
 
 	/**
 	 * Filters the value of the attachment's image tag class attribute.
@@ -2337,7 +2338,7 @@
 		</style>\n\t\t";
 	}
 
-	$size_class  = sanitize_html_class( $atts['size'] );
+	$size_class  = sanitize_html_class( is_array( $atts['size'] ) ? implode( 'x', $atts['size'] ) : $atts['size'] );
 	$gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";
 
 	/**