You actually don't need any :P just use the wifi dns method and your good but you probably wanna use BootMii, so i'd reccomend 4gb‹

TECHNICALLY you don't even need an sd card to do it, although it's highly recommended. i personally recommend 1 gigabyte at minimum for the nand backup

and by at minimum i mean at minimum. you'll probably need quite a bit more for other stuff as well

thank y'all so much ^^

i never mod a wii before, so idk what is bootmii... but i'll google it ‹3

8 gigs is perfect for all the apps you want but 4 also works