Format 64GB or bigger with
Guiformat to FAT32 (assuming you are using Microsoft Windows – other OS will offer formatting to FAT32 with their own tools).
There is no real limit for SD size. In theory 2TB are usable on MBR partition table with 512 Byte sectors.
For 128GB or bigger you can use 64KB cluster size to speed up load time when turning on. Huge SDs might lead to some waiting time.
Remember the overall 300 title limit. You can work around this with multiple SDs as well as 3DSBank.
Good luck and have fun!