PRINCIPLES OF DESIGN {SCALE & PROPORTION}

Proportion and scale: are usually expressed in terms of the size relationship of parts to one another and to the whole. Proportion also deals with shapes and forms and their dimensions. When the relationship ratio is pleasing, the furniture is well proportioned and fits the scale of the room. 

When furniture is off scale by an obvious amount, you almost feel like Alice, from Alice in Wonderland. In Alice and Wonderland, Alice goes through many drastic changes in height in order to get past the obstacles on her journey in her dream world. This however, leaves her too tall in some places and extremely small in others, and this made her feel uncomfortable. Imagine sitting on a couch where your legs can't reach the floor. You would feel smaller, and perhaps uncomfortable because the piece of furniture you are sitting on does't "fit" you. (Given you are in the range of an average height for a human)  To avoid all of this in your home, you essentially don't want to feel like your furniture is dominating the space, or in opposite cases, not "filling up" the space.

Classic examples of disproportional furniture in rooms:

The coffee table in this living room is disproportionate.
I don't see how guests who are sitting around such a massive table
can even get in their seats, because this table is taking up all the walking space! 

This bed seems to be enormous in this room because it's filling up almost all of the space.
A smaller sized bed in a smaller room would have this room in a better scale.  

Does this room seem sparse to you? That's because it is!
With the scale of this room being so large, the furniture here is not big enough to really "fill up" the space.  

The most basic method to achieving the right proportion and scale for a room through furniture selection is to measure, and "try out" your furniture before making a purchase. By measuring your room and the furniture piece, you will be able to estimate the amount of space it is going to take up when you add it to the room. And by "trying out" your furniture, you can determine whether the piece "fits" you. -As some pieces may work better with different people because of weight or height differences. So the next time you are decorating in your home, keep an eye out for proper proportion and scale!