How to Use SD Cards Bigger Than 32GB
Nintendo officially only supports memory cards up to 32GB -- no matter whether you're using SD or microSD cards and which 3DS iteration you own. There's no guarantee a larger one will work, though there are workarounds that make even larger cards function just fine.
How to format 64GB cards or higher for 3DS
For PC:
You'll need software like MiniTool Partition Wizard or comparable programs that support FAT32. Format the memory card with the FAT32 file format and create a partition with the cluster size set to 32KB. Set it to the primary partition and your card should be usable.
For Mac:
Connect the SD card to your computer. Open Disk Utility. To do this, click on the magnifying glass in the top-right corner of your computer screen. Type “disk utility” in the search box that opens, then click on the “Disk Utility” Application that comes up. Find the SD card in the left side of the window. It’ll be named something like NO NAME or UNTITLED. Click the ERASE tab in the middle of the window. Next, click on the “Format” dropdown menu. Select “MS-DOS (FAT)”. A message will appear asking, “Are you sure you want to erase the partition “[YOUR SD CARD NAME]?” Click “Erase”. Now the computer will delete the contents of your SD card and format it. You’re all done.