Most ladies' watch bracelets will fit up to a 7 to 7.5 inch diameter wrist; most men's watch bands will fit up to an 8 to 8.5 inch diameter wrist. Some dive/diver watches are even more generous, as they are designed for use while wearing a wetsuit.
To make sure you fit your new watch fits your wrist, use a soft tape measure to measure the circumference of your wrist. The tape should be pulled tightly around your wrist. After you have determined your wrist circumference, add 1/4 to 1/2 inch for extra comfort, depending on how lose you like to wear the watch.