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Officially upto 32GB however you can partition the card or use a program on your Pc called guiformat which will force format the full 64gb as one file but in FAT32 hence the upto 32gb limit
I use a SanDisk 64GB micro-sd card in my D5316 Sony T2 ultra.
I have been able to use the card in all 3 of my Xperia's. I own the T2 Ultra D5316, Ion LT28i and the X10.
I have never had to do anything other that format the card in my Ion when I bought it.
A screen shot from my Xperia T2 Ultra W/64GB storage
I swap the card between the 3 devices and have no issues.