Comment 11 for bug 1272057

Changed it to use DEFAULT_ICON_SIZE to make it less
error prone if the default size changes again in the
future. Pushed to master in:
http://git.xfce.org/xfce/xfdesktop/commit/?id=5203d9aa93e40dbddc81895fa60c8d1c99dd95f3
Thanks for spotting that!