Oh, can I weigh in for getting a Lab, even though it sounds like you would prefer a smaller breed?
I too live city-centre, with a park across the road and a small garden in the back. My Lab is only a puppy at the moment (got her at 7 weeks, and she is now 4 months old) so I can't talk about adult size and weight of a Lab, but I can sing the praises of her affectionate nature, trainability, and great behaviour around children.
I adore her. She is beautiful, intelligent, eager to please, cuddly and full of love for everybody, and extremely gentle around children. Even at her young age: she'll jump on adults to get high enough to their hands for a cuddle and a lick, but she sits gently before children and lets them maul her, and will sniff and gently lick babies if allowed, but would never pounce on or mouth them.
Thanks to the back garden, she was potty trained in a day, and learned sit-down-fetch in a week. Labs really really will do anything to please their owners, that's what makes training so easy.
Labs are wonderful. (I'm sure other dogs are too, and that the individual dog's personality is more important than the breed, etc. But get a Lab! You won't regret it.)