The maximum sized SD card the 3DS system is capable of handling is 2 TB (terabytes) under the FAT32 format specifications. It'll be slower the bigger your card is.
Your card meets the
minimum 128 GB capacity threshold. That means your card may optionally be formatted in FAT32 +
64 KB cluster size instead of the generally recommended 32 KB cluster size if you find the 32 KB one too slow.
The install limit is 300 titles per a single
Nintendo 3DS folder if available free space is not a limiting factor. You can get around this using either EmuNAND (not recommended) or GM9 scripts that switch/swap folders.
For more information about large capacity SD cards and game hording, here are several fine thread selections that go over these subjects: