As an alternative, try this:
1 – Locate the folder of the theme you are currently using. Example : the folder of the theme “Nuvola” in a standard gnome installation under Ubuntu is ‘/usr/share/themes/Nuvola’.
2 – Find the file ‘gtkrc’. If you find more than one gtkrc, choose the one that is “compatible” with the gtk version in use. Example: I used the file ‘/usr/share/themes/Nuvola/gtk-2.0/gtkrc’.
3 – Open the file ‘gtkrc’ with a (simple) text editor (or right click, choose scripts then gedit-root)
Example:’ sudo gedit /usr/share/themes/Nuvola/gtk-2.0/gtkrc ‘.
4 – Edit the line whose first field is gtk-icon-sizes so that the panel-menu field has the icon size that you desire. Example : I modified the line that reads : gtk-icon-sizes = “panel-menu=16,16″
into
gtk-icon-sizes = “panel-menu=24,24″.
5 – Save the gtkrc file and exit.
6 – Logout from your desktop. Log into your desktop. The icons in the menu should have a largerer size.
Worked for me - but I prefer the smaller size
Hope it helps.