Sorry, forgot the pics.

I've wondered whether I should have used arched mirrors to echo the arches across from that wall.

I think they are too high on the wall and small.

I would go with one larger, longer mirror with a more detailed frame. The straight, clean lines of the chairs and table give the room a hard, unforgiving look with the plain, small mirrors. A larger mirror with some "schmaltz" will pull in the style of your chandelier and rug and be more in scale with the room and table. Don't hang it too high... I would also consider a large floral painting with a lot of warm colors and a lovely frame.

So I was right. It looks too hard. How do I soften the look? I bought a beautiful antique display cabinet and found an antique mirror to match for the short wall that leads into the butler's pantry. Pic below. The scale of the long wall has me stumped. Should I not use a mirror because the room already has one? How many pieces of artwork! How big? The second pic is of the painting from the hall. I know it's too low....I hung it on the nails for the mirror. Size is 40 inches long by 34 inches high. It's a big wall!

The picture does not look like it is hung too low to me....

I really like the new cabinet and mirror, great finds

My thinking is a diptych or triptych on the wall. I'd stay with landscape shapes , and maybe even a nice landscape. Maybe a romantic era painting. Turner or constable would be nice. Look for something with some rich blues, violets and scarlet

Can I use abstract art in such a traditional room?

The picture is at a nearly perfect height as far as I can tell. It could be a little larger, but this one looks nice. Abstract would be fine as long as the frame is heavy enough and has softer lines. Always pick art that you truly love for the art itself and never apologize for it. And while I love traditional art with soft lines and rich colors, an abstract could make a cool contrast to the antique mirror and cabinet.

If the scale of the wall compared to the artwork still bothers your eye, try stacking two smaller pictures or even a couple of platters closer to the window in addition to the picture centered on the table. It will soften the feeling too. You might need a couple of people to hold them up for you to see where and if you like them.