Comment 9 for bug 1281368

@JohnLea, ah, nice if you want to increase the Shadows size as well...

You basically have to edit the variable defined in /usr/share/themes/Ambiance/gtk-3.0/apps/unity.css and switch theme (doing Ambiance -> Radiance -> Ambiance) from unity control center (unfortunately gtk doesn't notify us when a theme file has changed, so you have to force this in this way).