To keep scale in balance, use elements - be they furniture or architectural elements such as windows and doorways - that are in appropriate scale to the size of the room.
The scale of any part is most easily perceived relative to other parts of known size. Thus, the proportion and scale of elements is most clear when there is a hierarchy of parts; where large pieces are formed out of clearly repeating smaller pieces.
Space should compliment human sense of scale. Scale is percieved by relative size, often relative to human. For example when we see a doorway, we use it as a mental measuring stick by which to gauge the size of other things that we see and expect to be in proportion to the doorway.
Because we perceive things in relation to what we believe the scale of other things to be, our perception can be manipulated - for better or for worse - by playing with scale.