Odd numbers work better than even, and dissimilar sizes can be balanced by making sure that the middle of teach piece is at the same distance from the floor or ceiling. Also, make sure that the middle is at eye level. It is more interesting if there is unequal wall space rather than trying to have the same amount of wall showing both above and below the art. You could put the large artwork from your dining room over the middle of the sectional, add a smaller one on either side, and then stack a couple of smaller scale pieces vertically on the dining room wall. If the mirror is only reflecting the wall opposite, perhaps you can move it so that it either reflects light into the room, or is utilized as a last check before heading out the door.

It needs a well planned gallery or a huge picture.
My first thought was a big picture.

Create two areas to hang art, the dining area and the seating area. The lovely mirror seems to not be connecting to either area. The art above the sofa should be at least 2/3 the length of your sofa for better scale. Try to hang art gallery style, the middle of each piece should be 57 inches up from the floor. This method works great, it takes some measuring for each piece, but when you have more than one area to hang art, it's very easy for your eyes to travel around the room, because your eyes are at the midpoint of each piece. If you create a gallery above the sofa, put the biggest, heaviest looking piece on the left side, your eyes naturally go from left to right.

When hanging art above a sofa, or any piece of furniture, the 57" rule does not apply. You want the sofa and the art to read as a single unit, so hang the art 6 - 8" above the back. I would lose the mirror in the center. You don't have to fill the whole wall.

The wall appears to be three different colors. I would paint it all the same.

Lower the art in the dining area as well. It should be eye level when sitting at the table across from it.