Scale, simply put, means you should put like with like. If you have a large sofa and chair, then a small coffee and end tables will look silly. Put small furnishings together and large furnishings together.
Details, details. It’s the smallest items that can really make a room sing. They are the piccolo in the orchestra pit that sings like a bird in the trees. Accent trims on curtains, coordinating fabrics, repetition of design features, plants, and accessories, all combine to create a fully synchronized room.