I use only a 4GB (3990MB) hardfile with one partition DH0
This is enough for me for install Workbench, some demo and about 800 prefered WHDLoad games.
In order i have a drawer with classic PD games and others like Road Avenger, Heart of Alien Redux and TimeGal.
If i need more space i can add a second 4GB hardfile partition. Kick 3.1 with updated FastFileSystem support 8GB max.
I never tried to use a partition on SD Card as a real amiga harddrive.
I don't need it, but you can try. With Kickstart 46.143 you have no limit, thanks to the updated scsi.device directly in rom.