Apple Watch straps come in two size families: 38/40/41mm and 42/44/45/49mm (Ultra) always match your watch's case size. Bands are interchangeable within a family, so one watch can suit the gym, office and a night out.
Material matters: silicone/sport bands are sweat-proof and perfect for workouts, nylon/velcro straps are breathable and adjustable for all-day comfort, and leather or metal bands dress the watch up. For active use, look for a secure clasp and easy-clean material. Having two or three straps lets you switch looks in seconds while getting far more wear out of your watch.