The max size a 3DS ROM can be is 8 GBs. Currently, the largest sized ROM dumps are 4 GBs, meaning that you will need a microSD card larger than 4 GBs to get started.
However a GBAtemp user by the name of 3DSGuy has created a tool that trims 3DS roms to a smaller size that would allow someone to make a 4 GB 3DS rom small enough to fit onto a 4 GB microSD card.