Scale, the size of one thing in relation to another, and proportion, is everything to creating a visually pleasing environment. The most expensive and exclusive fabric in the world will be wasted if the finished project is out of sync with the rest of the room.
A balanced room design hinges upon three physical dynamics: the architectural scale, i.e. its structural proportions and characteristics, ceiling height, etc.; the human scale, i.e. the way in which people will use and interact with the space; and then there is everything in between, which creates a harmonious relationship between the two, leading the eye between them.
This has to do with manipulating the interplay of vertical and horizontal lines, and furnishings play an important part in this: the volume and positioning of curtains, screens, pictures, mirror heads, chandeliers and other light fittings can make or break the visual comfort of a space.
For instance, long tails on a pelmet will draw the eyes downwards whereas short tails will stop the eye towards the top of the window; layers of drapes held back at varying points will bring the eye to focus on those points, thereby breaking a tall and potentially overpowering vertical line.