You need an SD card of at least 32GB formatted in FAT32 format. You can not run games directly off your SD with Aroma as there is no Loadiine, but you can install them to a USB flash drive, hard drive, or the NAND on your console, so you don't need a massive or expensive SD, as most games on the Wii U are under 16GB, and once you've installed them you don't need to keep the folder on your SD anyway.