To make the battery icon smaller, open this file in a text editor:
~/.config/gtk-3.0/gtk.css
Then add this code:
#xfce4-power-manager-plugin image {
-gtk-icon-transform: scale(0.75);
}
Finally, save the file and redraw the panel:
xfce4-panel -r
Edit: It’s possible to transform the other tray icons, as well. For example,
#xfce4-notification-plugin image {
-gtk-icon-transform: scale(0.85) translateY(3px);
}
#pulseaudio-button image {
-gtk-icon-transform: scale(0.9);
}
#xfce4-power-manager-plugin image {
-gtk-icon-transform: scale(0.75);
}