Gap between wall flange and cement board
It looks like you might have installed the valve too deep in the wall.

The tile goes behind the silver escutcheon and up to the gray plastic circle that you can see from behind.

Usually the face of the gray circle template is at the same plane as the face of the finished tile. What kind of rough in valve is that? We can look at the instructions and it'll say. 

The round hole in your backer board isn't too big, that won't be a problem. The problem looks like you recessed the valve too far into the wall, and now the tile won't slide in between the silver escutcheon and the backer board.