Start with a simple tape measure, a notepad or your phone, and a helper if you have one. Sketch your patio, balcony, or rooftop as a rough rectangle, then measure the longest length and width at floor level. Take measurements in a few spots, because walls and railings are rarely perfectly square. Note anything that affects the usable area, such as doors, planters, drains, air-conditioning units, columns, and railing height.
Next, measure the path furniture will travel through: the front door, hallway, elevator, stairwell, gate, and any tight corners. A sofa that fits on your balcony is no good if it cannot get there. Also check vertical clearance and overhead obstructions if you are considering an umbrella, pergola, or tall seating.
Do not design around the full footprint. Leave comfortable walkways, usually around 24 to 36 inches, and allow about 30 inches of pull-out space around a dining table. For loungers or reclining chairs, measure how far they extend when fully open. Use painter's tape to mark the furniture footprint on the floor so you can see how it feels in real life.
Finally, compare your usable measurements with the product dimensions online, including legs, arms, and reclining positions. Check weight limits and drainage or slope if relevant. Order only when the furniture fits the space and the delivery route. A few minutes with a tape measure can save you the hassle of returning oversized urban outdoor furniture.