An effective decoration of a room largely depends on its size and shape and mainly the purpose for which it is going to be used. Living room decoration can be either a simple task or a complicated one depending on the people that are going to use it.
Simple if it is going to be empty for most of the time except when the family is at home; complicated if a lot of entertainment is going to take place. But that does not mean that it should not be the opposite also. It largely depends on the person who is going to decorate it. Another complication would be if a constant stream of children and pets are going to be traversing the room.
The shape of the room also has a big say in the kind of décor that it can take. In a large room with plenty of space, it can be segregated into some definite functional space. A small office space beneath the stairs or a formal dining area in a separate place by itself. The use of colors also has a telling impact on the atmosphere of a room. If not judiciously used a large room may look small and cold; if judiciously used a small room can be made to look larger and warmer. The wall art and other accessories in the living room sets its ambience. The more uncluttered the room the more comfortable it is to live in and spend time.
Keep picture sizes small wherever possible, or maybe a large one in one of the larger walls. Avoid substantially large pictures as far as possible. A two-seater settee, rather than a three-seater will create an impression of space. Mirrors are good accessories as long as they are placed in strategic positions as they refract light around the room.