diff --git a/ace-window.el b/ace-window.el
index ccb2783..239f909 100644
--- a/ace-window.el
+++ b/ace-window.el
@@ -142,8 +142,10 @@ Its value is an (x-offset . y-offset) pair in pixels."
 
 (defcustom aw-frame-size nil
   "Frame size to make new ace-window frames.
-Its value is a (width . height) pair in pixels or nil for the default frame size.
-(0 . 0) is special and means make the frame size the same as the last selected frame size."
+Its value is a (width . height) pair, where width is in characters and
+height is in text lines, as passed to `make-frame'.  Nil means use the
+default frame size.  (0 . 0) means match the size of the currently
+selected frame."
   :type '(cons integer integer))
 
 (defcustom aw-char-position 'top-left
@@ -467,7 +469,7 @@ LEAF is (PT . WND)."
             (cons 'height
                   (if (zerop (cdr aw-frame-size))
                       (frame-height)
-                    (car aw-frame-size))))
+                    (cdr aw-frame-size))))
           (cons 'left (+ (car aw-frame-offset)
                          (car (frame-position))))
           (cons 'top (+ (cdr aw-frame-offset)