I’ve followed https://github.com/kth5/archpower/wiki/Installation-%7C--NewWorld-PowerMac-with-Grub to install GRUB, but the recommended 4 MiB is waaaaaay too small, the 12 MiB in the screenshot is just barely too small, and 32 MiB had a weird wraparound where it said it had a negative number of sectors and then GRUB failed to boot, leaving me in a rescue shell.
With 16 MiB it worked fine, leaving 3.3 MiB free for GRUB to grow a bit over time.