If you are setting up a personal 3DS archive, here is how to approach it:

There is no single "official" size for a complete 3DS ROM set because the total volume changes drastically based on regional variants, file formats, and whether eShop exclusives are included.

There are approximately 1,300 to 1,500+ unique 3DS titles .

These are direct dumps from physical retail cartridges. They mimic the exact size of the original game card (e.g., 512MB, 1GB, 2GB, or 4GB), even if the actual game data only takes up a fraction of that space.