What factors determine fuselage cross-section design?

Fuselage cross-section design considers: Pressurization - Circular or near-circular cross-sections distribute pressure loads efficiently (double-bubble adds floor area at weight penalty); Passenger comfort - Seat width, aisle height, overhead bins, and class configurations; Cargo capability - Belly cargo (LD3 containers, pallets), main deck cargo; Structural efficiency - Minimum wetted area for given volume, simplified manufacturing; and Aerodynamics - Fineness ratio (length/diameter) affects drag, typically 8-10 for transports. Single-aisle aircraft (A320, 737) typically seat 6-abreast, wide-bodies 7-10 abreast. Cross-section determines cabin flexibility for different configurations. Larger diameter adds weight and drag but enables more passengers and cargo. Recent designs (A350, 787) use wider cross-sections for passenger comfort.