I would wait until the dog is at least 6 months. I know especially for male Rottweilers (and it's probably true for other large breeds), that if you want them to fill out completly then you have to wait until they fill out (at at least 2 or 3 years of age) and then get them neutered. But if yours is a female, then you probably shouldn't have to worry about that because they're never usually as big as the males. I would definatly wait until 6 months, but then again, I'm not a vet!