Odd this hasnt been answered before now but my guess is 64 GB is correct but to err on the side of 'safety', 32 GB is suggested as its been tested and found to work and its also quite cheap and easily available.
Another point to consider is that using a 64 GB is a bit risky because if anything did go wrong with the card or you loose it, thats one hell a lot of material GONE-GONE-GONE.
Based on a 4.5 GB per Hour of Video recording (forget how many happy snaps that is) 32Gb gives you over 6 hours of video recording and thats a lot of crap holiday viewing.
I did read sometime ago there was a 256 GB card available, NOW thats really a lot................
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