You probably haven’t thought about the SD card that came with your Nintendo 3DS since you bought it. In most cases, it came with either a 2GB or 4GB card that was already in the console, so you didn’t even have to think about it. But I assume you are here because you’ve ran out of space on your card.
The 4GB or even the 2GB sized card is normally more than enough for your system files and the images your 3DS takes. Because of the 640×480 resolution, the images taken by your Nintendo 3DS average about 100kb each. Even if you take the occasional video, the usage is fairly negligible as long as you don’t use it excessively. You can add a few music playlists and have plenty of room leftover. You can more or less ignore this part of the equation and maybe allocate 1GB for everything you’ll ever need.
Almost the entire space of your SD card should be dedicated to downloaded games from the Nintendo eShop. They use a bizarre block system that equates 8 blocks to 1mb and 8,192 blocks to 1gb. On average, a Nintendo eShop title that was meant for the Nintendo eShop is about 32mb each; once again fairly negligible even with just a 2GB SD card.
The trouble begins with retail downloads. These are full downloads that replace the cartridge games. In general, the average game is roughly 512mb each. However, RPG titles are known to be quite a bit larger, generally ranging from 1 to 2gb. A few higher production games push that limit further by approaching 4gb, these include Resident Evil: Revelations, Bravely Default and Donkey Kong Country Returns. As you can imagine, as developers continue to create games for the 3DS, they will get larger and more complex. It should not be surprising to see more and more games use 4gb of space.
So with all this in mind, what size SD card should you get? You need one of 2GB at least, that will give you 1GB of space for your system files, images and sounds. It will also leave about 1GB free for any game patches that might occur and a few smaller eShop titles. If you intend to purchase just a few titles from the Nintendo eShop, it might be a good idea to buy an 8GB SD card, that will give you room for a high production game and a few smaller titles as well. If, however, you’ve given up on cartridges in general and have opted for digital downloads altogether, I’d definitely recommend making the step up to 32GB or even 64GB.