I'm in the middle of a bathroom reno and looking to pick out mirrors.

I have sconces on each side of my mirrors, which means I need to use a somewhat slimmer mirror (18-20" wide). I found two that I'm considering but not sure how well the proportions fit. The ceiling does vault up to 10' so maybe it would look nice with a tall mirror. I'm just worried the taller option (20x42) won't look proportional with the 18" wide tall cabinet on the right. I know a mirror shouldn't exceed the width of the piece it is under -- wasn't sure if there is any rule of thumb vertically when next to a cabinet. Pictures below (cabinets will essentially go on right where box is).