actually it is theorically 2TB because it is the maximum limit of the SDXC standard that the 3DS use, so they do not exist to this day but they will in the future, and once we want to get bigger than 2TB we will need a new standard (SDUC) that will not work with the 3DS
ps: well its also the limit of fat32 partition size, but thats not what matter here, if that was the problem you could have a 4TB card with a 2TB partition only
ps2 for info:
SD standard was up to 2GB
SDHC from 2 to 32 GB
SDXC 32GB to 2TB
SDUC 2TB to 128TB
ps3 : when i say "the SDXC standard that the 3DS use" i of course mean by that that the 3DS can use UP TO SDXC (so SD and SDHC work too)