I told everyone and also had my parents tell my immediate family (I don't live in the same state as then but I visit often). I wanted everyone to know because I'm a very open type of person, but I also know how my family is and they can be very gossipy and/or start assuming things (like I'm sick hence the weight loss), so I decided from the beginning that I wanted everyone to know. It hasn't been a secret to family and friends that I've been overweight my whole life, and I knew having this surgery would make the difference, in the sense that I finally got the weight off, so I knew I'd eventually get asked what made the difference this time, and I didn't want to make it seem like I was hiding something.
Good luck!