Better question would be what can you afford? If you can afford a 128gig card then you can use that. Just remember it's best to use a quality sd card and not the cheapest. I'm currently only using a 32gig card but looking to go 64 or if I can swing it 128. I load lots of music on mine.
lol When the phone came out there were no cards bigger then 64gb. 128gb cards will work. They just were not out when the specs for the phone were released.