A side table has one job, which is to hold a drink and a phone within reach of the person sitting down. That makes the sofa arm the reference point, not the room. Measure from the floor to the top of the arm and stay within about two inches either side of that number. Most sofa arms land near 22 inches, which is why the 20 to 24 inch band covers the majority of living rooms even though end tables are sold anywhere from 18 to 28 inches. If you are between two heights, take the shorter one. Reaching slightly down for a glass is natural. Reaching up and over an arm that is lower than the table is how drinks get knocked. For an armless sofa or a low slung modern frame, use the seat height plus a few inches instead, and expect to shop shorter than you assumed.

Shape is the difference between a table that fits and a table that catches your shin. A round side table has no corners to walk into, which is why it works in a tight gap beside a chair or at the end of a walkway. It also gives you less usable surface than its diameter suggests, since anything set near the edge feels precarious. A drum or pedestal table has a solid base, which reads heavier and gives you a full top with nothing to kick, and it is the most stable choice on a rug. A C table slides its base under the sofa so the top hovers over the seat, which is the best answer for a laptop or a plate. Nesting tables are the flexible option: one stays put, the other gets pulled out when there are more people than surfaces, then disappears again.

Look at the floor before the finish. A three legged table on a thick rug will rock, and a slim pedestal on a hard floor slides when you lean on it. Between a sofa and a wall, depth matters more than width, so a small round table beats a square one every time. In front of a window, a low open frame keeps the light. Beside a chair that people get out of often, avoid a sharp corner at knee height. Material follows use in the same way. Stone and sintered tops take a wet glass without a mark and are heavy to move for vacuuming. Wood shows rings unless it is properly sealed, and shows them beautifully after a few years. Woven and rattan tops are the lightest to shift and the worst place to set a dripping glass. If the table will hold a lamp, check the top is wide enough for the lamp base plus a drink, which is usually about 14 inches.

If you take one thing away, write down your sofa arm height and refuse to shop outside the four inch window around it. That one constraint eliminates most of the market and nearly all the regret. After that, shape is a practical choice about circulation rather than a decorative one, and round wins more often than people expect simply because nobody bruises a knee on it. We would put a C table in any room where people eat or work on the sofa, because it solves a problem no other shape solves. And we would resist buying a matching pair for both ends of a sofa. Two tables of the same height in different shapes look considered and let each end do a different job, which is closer to how the room actually gets used.