You would get a little less than 64 GB at your disposal, but don't; you should only use HDDS, preferably adapter-powered. Flash drives are not something you should be thinking about for Wii U.
1. Nintendo warns against it.
2. There might not be enough power to consistently read/write and you'd need to use a Y-cable anyways, making it quite complicated for a simple flash drive.
3. No one knows how often read/writes are made to Wii U connected mass storage devices. because of this, the wear levels my exceed what they would be in a normal application as far as flash drives are concerned.
So if you want to use a flash drive, don't. Too many people have been fucked over by the, some on GAF even. If you ARE gonna use one, make sure you keep a duplicate to regularly back up the main one. If you really want peace of mind, just get a good powered HDD.
http://www.amazon.com/dp/B008DW96NY/?tag=neogaf0e-20
This one not only complements the Wii U very nicely, but it is officially endorsed by NOA.