From Sony's support page:
https://www.sony.co.uk/electronics/s...icles/00171492
he maximum capacity for micro SD cards for use with the NW-A3x/A4x series Walkman players is 2TB.
All Sony micro-SD cards currently available on the market are confirmed to be compatible.
NOTE: Sony cannot guarantee full compatibility with cards from other brands.
They won't guarantee compatibility woth all SD cards, because they couldn't possibly test every make and size, and there are so many fakes out there, but you notice it certainly didn't say the device only works with Sony cards.
Reading further in the online instructions, it appears that the file structure for the device is quite specific, and you in all likelihood have to format the card and then load files using their app and dragging and dropping. Not uncommon on various products of different manufacturers. You could probably take a (smaller) SD card, format it, and load some music onto it using the Sony software on your PC, with the card in the player. Then you could quite possibly insert the card (with an adapter if necessary) in your computer and using the properties function determine what file system is used (FAT32, etc) and using Windows Explorer, see how the file and directory structure is organized, which might (MIGHT) allow you to copy files to the card in your PC without using Sony's software.
It is pretty evident that any size SD card will work.