If it does 64, it will almost certainly do 128 and higher.
To do 64 it has to support the SDXC standard, which will take you to 2TB cards. If they say 64G that probably just means that's all they've tested.
One caveat - I don't know about current models, but when I had a Hero 2, it was the absolute pickiest camera I've ever owned on SD cards. I had a dozen cards from many manufacturers, they all worked 100% fine on every camera and other device, except for the GoPro, which only worked right with 2 cards and even then did something to them that made them then not work right in many other devices.
When I asked them about it they got real snotty and said I should stop buying crappy cards. 2 of the cards that didn't work right were the exact SanDisk model that they recommended.
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