I'm looking to see if I can reduce the height of the wingpanel in elementary OS Freya. I mean make the wingpanel smaller like normal wingpanel in elementary OS Luna.
You can make the panel shorter by editing your system's stylesheet. If you haven't modified your system, the file you want to edit is
/usr/share/themes/elementary/gtk-3.0/apps.css. You can adjust the height of the panel by reducing the padding in the selector
.composited-indicator. The current version of the stylesheet doesn't style
.composited-indicator directly, so you'll have to add a line for it.
.composited-indicator is a Gtk.MenuItem so it inherits all the style properties defined in
/usr/share/themes/elementary/gtk-3.0/gtk-widgets.css under the selector