Check out this link, very helpful for making large cards work:
http://lofi-gaming.org.uk/blog/2013...-upgrade-your-3ds-sd-card-to-64gb-and-beyond/ I'm using an 128GB on an O2DS, so don't worry about the size, format it right and there won't be any issues.
Format the card to fat32. Then drag all the files from the old sd card to this one. As long as you don't use an emunand (which if you have no clue what I'm talking about you're fine) everything will carry over to the new sd card with no issues. You really don't need anything over 64gb since the 3ds has a 300 game/app limit, but there are a few exceptionally big games.
Considering there are a LOT of games that take 2+GBs each, it really fills up fast! Get the Pokemon games (7-22GBs depending on how many), the Monster Hunter ones (3U is 2.2GBs, 4U is 1.6, Gen is 2.1 and XX is 2.4), Dragon Quest 7-9 (each is 2.8-3.3GBs), Resident Evil: Revelations (3.1GBs), Zero's Escape: Virtue's Last Reward/Zero Time Dilemma (3+GBs total), Donkey Kong Country Returns 3D (3.5GBs) and so on and it will fill up before you know it! Also, is there really a 300 app limit? I have a LOT of VC games, enough to be past that...
