If you still want to know, I am using a 128GB card for usbloadergx and nintendon't. I originally had it in the sd card slot but usbloader gx does not load wii games from sd so I was using wiiflow and everything worked fine. Eventually I grew tired of wiiflow and moved back to usbloader gx using the sd card with a usb adapter.
I bought this card for about $40 and have had no problems with it. As a bonus it came with the usb adapter that I am using.
http://www.amazon.com/SCT-Digital-Ultimate-Extreme-S-F128-RT-550/dp/B00EOJ8HTE/ref=sr_1_6?s=pc&ie=UTF8&qid=1422885079&sr=1-6&keywords=128Gb sd card
Also when using the adapter you can ustealth the card so you don't get the format drive message from the wiiu.
As far as I know you can use any size card you want in vwii but to do the original hack i.e. smashstack you need a 2gb or smaller card.
Edit - I only use this card for wii games now. I moved all of my gamecube games to another sd card that stays in the sd slot. Nintendon't has issues with having two usb devices connected. My wiiu hdd and my sd card adapter used for usbloadergx and wii games.
So I have 3 storage devices connected to my wiiu.