When it comes to wall decor, its size matters and should be between roughly two-thirds the size of your the object that painting is sitting above. When hanging artwork over a sofa or other large piece of furniture the bottom of the picture should be atleast 8-10inches above the it.
You can use a single huge piece of art, two, three, or a cluster of smaller pieces. The art that should be at least two thirds the size of your sofa.
If using a collection of art pieces they should be placed approximately two to three inches apart, but never farther apart than six inches. By combining larger and smaller elements together creates a sense of interest and energy. This also functions when combining both vertical and horizontal pieces into one grouping.
Always hang the focal point of the piece or grouping at around eye level so that everyone can enjoy it.