Alright I will get a 128 SD card today ... Cheers for the information....
It's a bit late now, but just to let you know:
There is a 300 game limit on the number of games the 3DS can display through the stock menu. If you take the top 300 games for the 3DS (as defined on Metacritic) it uses up ~220GB of space once installed. So to max out your 3DS with the maximum number of good games, you would need a 256GB card (this would also leave room for saves/updates/DLC).