I currently use Ubuntu on my macbook pro with grub. My macbook has a retina display with 2560x1600 resolution. The grub startup screen font is so tiny that it is barely readable.

How can I make grub appear in an eye-friendly way with this setup? Just passing on a gfx mode like GRUB_GFXMODE=640x480 does not have any effect?

Thanks for a solution.