If you buy a card that is faster than what the N20U reads/writes then it'll be at whatever the max is of the phone. If the card is slower than it will be at the max of the card. Either way speed doesn't matter for the most part for compatibility. Size also shouldn't matter SDXC supports up to 2TB so unless Samsung gimped it you can go to 2TB. Unless our phone has SDUC card support then it's 128TB.