From what I've seen BBLoader should be more "iffy" about the USB adapter used, but the G-Boy kit comes with the recommended adapter so you don't have to worry there.
Beyond that, I can second the recommendation of Samsung. Their flash media just works and is cheap on Amazon. The Evo+ and Evo Select models are identical. The Green/white Evo Select is just an Amazon exclusive. If you're in the US at least you can grab a 128GB evo select for $20 with prime. I, and others, can certify its compatibility.
When it comes to size remember that you'll be able to take games off and on with the USB-C port, so you can swap when you want easily. Sizes of games vary but how big of SD you want really comes down to what library of games you want to carry around. 64/128's are cheap and 256 is probably WAY overkill for 99% of people. Others can probably provide better insight on the average size of GC/Wii games.
Also it looks like you're newish here. Welcome! If you're using an adblocker, you most likely can't see the Amazon link I added.