Galleries and museums hang artwork so that the midline (center) of each piece is 57 inches to 60 inches from the floor. (The average human eye level is 57 inches.) And you should do the same. In a room like this, where the ceilings soar, there might be a tendency to hang the art higher. But remember It needs to relate to human scale, not the structure scale.If you are not sure, take a picture. it is remarkable how much a photo can reveal. Print it out or use Photoshop or an app to draw on the photo. This can give you a sense of whether a larger or smaller piece of art is needed or a tall plant might be best to fill a vacant spot.