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

I run on 128 GB :)

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.

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

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

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.

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.

64 GBs, and it is far for being full lol.