I have a Sandisk Ultra 128gb MicroSD card, and it works fine.
You do have to be careful, though, as it's not uncommon for various cards of various sizes and manufacturers to simply 'disappear' from the Operating System at times. I had a Lexar 633x 128gb Card which did this. It mounted fine, but then when I was doing various operations, plugging in other devices through the USB port, it would simply unmount itself and be unavailable.
It isn't clear whether this is with certain particular Brands and Models, defective cards from various manufacturers, driver instability, or what. You can do some searching of this and other forums, as there is quite a lot of discussion on the matter.
I also found this on
Microsoft's website:
You can use USB storage or a microSD card of any size with your Surface to store your documents, music, videos, and pictures. Surface works with standard microSD card formats—microSD, microSDHC, and microSDXC.