You could highlight it by painting a section of wall behind it that frames the piece. So lets say for arguments sake that it is 1m wide, I would paint a band of wall from floor to ceiling in a darker colour that comes out 10-15 cm further on either side. Then hang the art work up and it will contrast beautifully and fit the freshly painted section nicely and make it seem more in proportion. The alternative is to create a gallery wall, so hanging more pieces of art in combination with that piece to create a collection that fills the space better. Good luck! and let me know how it turns out.

Could you make yourself a little clearer please? What is your question?

Also, I think the mirror will look better over the fireplace, with art on the radiator wall. I would paint a radiator the same colour as the wall or use a simple radiator cover. And a good tip for buying furniture: measure! Measure the room, measure the sofa (for example), tape the dimensions of the sofa out on the floor so you get an idea of the space it will take up. Furniture looks smaller in the shop than it looks in your room.

Please post photos and floorplan if possible.