Hi @Sergioigram,
I will not be able to reply if it works or not as I don't have a 256 GB SD card at hand, but hopefully you will find the information some what helpful at least.
The supported size of an SD card for Xperia devices that is stated in specifications or white paper is the size where quality and performance can be guaranteed. It may however be possible to use larger SD cards, but in such cases quality and performance cannot be guaranteed.
The white paper mentions that the Xperia Z5 supports SD cards up to 200 GB and that is what we can guarantee quality and performance, however a bigger SD card may still work.
Unless someone here has tried with a 256 GB SD card, do you have a friend, colleague or family member that have an SD card in this size that you try in the device, before you purchase one? I don't know about the return policy for SD card that have been tested, but you can always check with a few different retailers, in case it doesn't work.
I hope that helps a bit! 
Hi @jmaxim917, thank you so much for letting us know that it works fine in your Xperia Z5 Premium!
There should be no difference between the Xperia Z5 Premium and the Dual version of the Xperia Z5 when it comes to the SD card size, so it should work just fine in a Xperia Z5 Dual.
Thanks again and have a great day!
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