Sony Xperia Z supports 64GB microsdxc memory cards
Some suggested to us that it does, we just needed to format the card in the phone.
It turns this is correct, you can use a 64GB card. We have tested this on a final production unit.
As above, you do need to format the card in the phone for it to be read, so you will lose any data already on the card, but a 64GB microSDXC memory card will work.