I would suggest getting two gangable old work boxes or a two gang old work box. Buy the box and cover before you tile, and you will be able to hold a ruler up to them to determine the size of the hole you need.
Your 2G box is fine. A mud ring goes on a 4 square box.
Be sure and cut your tile allowing room for the GFCI/decora switch cover plate screws which are about 1/4 inch above the device screws. If you cut the holes too tight the screws tend to stop when the hit the tile. It is a very precise cut because you want the plaster ears to hit the tile but not the cp screw.
If you use nylon covers you can float the devices out a touch and the plates will secure them without breaking.