--- hw/xfree86/modes/xf86Crtc.c~        2013-10-03 09:54:41.000000000 +0100
+++ hw/xfree86/modes/xf86Crtc.c 2013-10-03 10:31:46.020266748 +0100
@@ -3097,6 +3097,15 @@
     xf86OutputSetEDIDProperty(output, edid_mon ? edid_mon->rawData : NULL,
                               size);
 #endif
+    if (output->conf_monitor &&
+            (output->conf_monitor->mon_width > 0 &&
+                output->conf_monitor->mon_height > 0)) {
+            /*
+             * Prefer user configured DisplaySize
+             */
+            output->mm_width = output->conf_monitor->mon_width;
+            output->mm_height = output->conf_monitor->mon_height;
+    }
 
     if (edid_mon) {