Shape and size: Squares should be 18 to 24 inches across, large enough to showcase a solid color or a pattern. Oblongs are a nice foil, since they’re “a little less expected,” says Katie Ridder, an interior decorator in New York City. Fabric: A limited color palette and large geometric prints on smooth, tightly woven fabrics, like cotton and linen, work well in a modern environment. But if you love having lots of colors, take a tip that applies to all settings: “Pull them from a piece of art that’s in the room,” says Andrew Flesher, an interior designer with Gunkelman Flesher Interior Design, in Minneapolis....