The official requirements for flashing the eMMC are "4GB or greater". You need enough room to store a bootable OS and the image that will be written to the 2GB eMMC.
If you're using the card as external storage while running from the OS on the eMMC, any size should work. The numerical upper limit on the FAT32 filesystem is about 8 terabytes, but the conventional upper limit is 32GB.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.