The size of art to place above a sofa should be somewhere between ⅔ to ¾ the width of the sofa itself.  For example, if you have a 6’ sofa, multiply 6 by 0.66 and 0.75, which gives you a maximum length of 4.5’ for your artwork.
The key to designing with art above your sofa is to consider proportion and balance. The sofa is most often the largest piece of furniture in a room, creating visual weight. To balance this, your artwork needs to be slightly smaller.
Consider adding one large piece of art, such as cheap oversized wall art, or two to three same-sized or different-sized pieces of artwork. As long as you fill up space equal to approximately 75%  of your sofa’s width, you’ll keep the balance and create the best look for the room.