5.5.1 update on Wii U side does not affect your homebrews on vWii side, as long as your files are present on your SD card it will run fine..on the other hand, Wii U homebrew stuff only consists of a few MBs but games like Xenoblade Chronicles X takes up to about 19GB+, Bayonetta 2 around 15GB+, some games like eShop titles have lower file size, example is Child of Light which is around 2GB+ only, some even less. So safest bet would be a 64GB card if you play a couple or if you want (nearly) everything in one card, 128GB would be the best choice..but i suggest 64GB, that way, you will be forced to focus on a couple if games at a time, and be able to finish it without jumping to another game and end up with lots of backlog games,but that's just me..