Let's assume a reasonably modern BIOS (2005-), with no bugs/quirks, 512 sector size, LBA48 addressing, and Enhanced Disk Drive support.
The reason for asking is that by a naïve reading of INT 13h Extended Read call, offset sector is an 8-byte value; at 8 ZiB larger than any disk available and beyond the reach of LBA48 addressing. Yet I only find reference to smaller limits, so what is GRUB actually able to support?
What size fields is the blocklist comprised of, used by boot.img / diskboot.img?
GNU GRUB's (also known as grub2) blocklist entries have a 64-bit (uint64_t) field for LBA numbers, so they can store 8-byte values. But the limiting factor will in most cases be what the BIOS INT13h Extended Read routines will accept: if the underlying hardware limit is LBA48, then the limit will be 2^48 blocks, which on 512-byte block size works out to 128 PiB, or about 144 PB.
If we're talking about GRUB Legacy (GRUB 0.9x), its blocklists can only hold 32-bit (uint32_t) LBA values.
On a UEFI system, the firmware disk access API limit is at 9.4 zettabytes, and as grubx64.efi can just tell the firmware to load a particularly-named file from a GPT partition identified by an UUID, GRUB's limits might not be applicable at all. Of course, if you use a filesystem type that is not supported by the firmware, you'll need to rely on GRUB's filesystem drivers, which may not (yet?) support filesystems of that size :-)
