Factoring scale into your design is very important. Scale refers to the relative size of something to another. (The lamp on the left is at a giant scale relative to a person).
For example: if you are designing a room with very low ceilings, furnish with low pieces. If you use very high furniture, it will make the ceilings seem even lower (because it reduces the space between the furniture and the ceiling), and the room will feel smaller. Lower the plane of the upper boundary of the furniture to allow for more air above it that is unoccupied. That equates to additional space.
By contrast, if you have massively high ceilings (like a double height room) using higher elements in the room will help make it look proportional. Furnish with higher-backed seating, tall floor lamps, and taller accessories. Then, drop in pendants from the ceiling that can bridge the gap between the floor and ceiling planes. This will unite the room and make it feel like one environment, rather than a room that expands infinitely upwards.
Use small scale furniture in a small room dimensionally - stay away from big, bulky pieces if you don't have a lot of room for them to breathe. Again, using smaller and lower furniture will make a room feel larger.