Magnify scale. The living room of O'Brien's Manhattan apartment has 20-foot ceilings but a small footprint. He humanized this potentially cold space by creating symmetry. Taking his cue from the room's double windows, he populated the room with pairs of furniture. Matched pieces allowed him to introduce fewer overall shapes, which enhanced the scale of the room. Tip: In small spaces, be rigorous about selecting furniture. For each decision, O'Brien suggests you ask: "Why this piece, why this place?"