I have a 32GB Card in my 3DS, personally.

I think 64-128GB is all you will ever need

i have 32gb in my wii u

highly depends.. i recommend at least 32GB

64g is honestly perfect, you can mess around and get all the gamed you want. Another thing is the more your card size is the longer the boot up is

I would get at least a 32, if you get anything higher though dont forget to format it to fat32 with a program as windows will not format an sdcard higher than 32gb to fat32. Personaly I am using 32gb as thats what I had lying around.