There is some discussion here: http://forum.xfce.org/index.php?topic=4676.0 but this work a little incorrectly -- the line: gtk-icon-sizes = "gtk-menu=24,24" in .gtkrc-2.0 caused all icons in system menu to be resized while keeping icons in menus of ordinary applications unchanged. Unfortunately setting the "/Gtk/IconSizes" causes normal menu icons (for example: icons in the terminal's menu) to be enlarged too. As a result: menus in normala appliactions are of *very* different sizes depending on icons they contain. Very strange and very annoying.
Er, why did you file two bugs about this? *** This bug has been marked as a duplicate of bug 5660 ***