Having the right scale of art on your walls will make a world of difference in balancing your room. We'll focus on hanging art over a sofa here.
Your sofa is usually on the largest wall, so even in small homes, there is quite a bit of space. Creating a focal point on that large wall is a great idea. There are are three tips that I'd like to share, and you'll decide which is right for you based on the type of art you enjoy or collect.
The first style is to create a gallery wall. That entails using different pieces of art with different frames, content, and color, although you will create continuity in some way. First, select the pieces you want to use. It's ok to fill the wall behind the sofa, leaving enough space at the ceiling and above the sofa so it doesn't feel crowded. I like to start with the largest piece first, and hang it off center so I can work other sized pieces around it. I hang other pieces so that similar colored works, or those with similar type frames, run across the wall rather than be grouped together. A good rule of thumb is to alternate the direction that each piece hangs, so that some are horizontal and some hang vertically. I do often hang similar sized pieces one on top of another.
Another way to create a focal wall over your sofa is with a single large piece. To create balance, I recommend that the width of your piece be about 3/4 to 2/3 the width of the sofa. If it's too small, it will seem out of proportion.
Finally, I also use pairs or triplets of art over sofas. Pairs don't need to match, but should have the same frame and similar content or color. They shouldn't hang too far apart, but be within 4-8 inches of one another to create a group. For triplets -triptych - the art is often one piece that has been cut into three and framed identically.