Harmony in interior design is all about balance and proportion. The scale of a room, either large or small, will have an impact on the overall layout. Some elements, like the width of pathways around the space, will have a minimum convenient size. Other choices, like oversizing the sofa in a large living room, or selecting a petite loveseat and glass-topped tables in a small living room will help make the finished design more harmonious. The goal after all the practical considerations have been dealt with is to create a unified whole in which all the pieces fit as if they belong together.
If this sounds a bit intimidating, it doesn't have to be. Most room layouts are works in progress, subject to change during different seasons of the year or for holiday entertaining or relaxation. The biggest rule, and the one that's the most important to remember, is that keeping a room easy and safe to use is always a top priority. For the rest, the more you play with room design, the more you'll learn about what looks balanced, attractive and suitable for your family and lifestyle.