I have a 256GB one. People say not to go above 128GB, but I just had this one lying around and honestly I've had no issues whatsoever.

Also I think there's a max of 300 games you can have installed at once.

My New 3DS XL systems each have a 256 GB SD card.

My old 3DS XL systems each have a 128 GB SD.

I run on 128 GB :)

For most of my time with my 3DS, I used a 4GB SD card, which I suspect came with the system itself. I almost never got non-demo games on the 3DS digitally, and most of the games I played weren't too abundant in the updates, so it was enough for my purposes, even if I was skirting at the edge of how much it could store. When I modded my 3DS shortly after hearing about the 11.17.0 firmware update, I figured I'd need an upgrade before modding it, so I went with a 32GB microSD card that was lying around in my house's storage tub, waiting to be used.

I've got a 128GB in my 3DS and a 32GB in an Acekard2i that's shared between the 3DS & DSLite.

I don't need either of those cards to be as big as they are, lol. Those cards had just happened to be big enough to fit my roms & not otherwise used in anything, so they're what I used.

I imagine an avid user could still easily get by with just a 32-64GB 3DS SD, and a 4-8GB SD for a NDS flashcard.

Edit: Oh, and my GBA flashcart's SD size is still somehow absurd for the amount of roms it has on it, while still only being 4GB in size.

I have 128 GB in my SD card slot and 64 GB in an Ace3DS+ card.