When your mac has a 15,4 inch retina display, you can add GRUB_GFXMODE=2880x1800 to the /etc/default/grub file, than run sudo update-grub, and reboot.
Next you have to find lines in /usr/share/glib-2.0/schemas/org.gnome.desktop.interface.gschema.xml file 1 and change the scaling factor