Always, always, always adopt. Shelters do their best to match the dog to the owner. If there's a trait the dog has that wouldn't fit your situation, the shelter will find you another dog. "Unpredictable" dogs don't get put up for adoption. As for size, any dog over 2 years is probably full-grown or close to it. Also, aside from medical reasons, don't have your heart set on a single breed. My mom always said she would never have a pit bull, not because she was afraid but just because she liked other dogs better. And yet here we are, with an adorable little pit/boxer mix, and she's the perfect dog for us.