1. No dog is easy to train, you'll have to devote some time to training a dog, even if it's the smartest of breeds like the Poodle or Collie
2. Some dogs are tolerate to heat or cold, you should research this before you get a dog? What kind of weather would you say you live in?
3. All dogs can be sweet, larger dogs are usually tougher and are able to withstand more.
4. All dogs will be alert when they see strangers or most dogs, but no dogs is guaranteed to protect you, unless the dog is a certified PPD dog, and certifying or getting a dog trained in protect can cost $10,000 or more, yes! that much.
5. What kind of living environment do you live in, small or big? Smaller dogs do better in smaller environments, bigger dogs do better in bigger environments, if you want a big dog, as you say you don't want a dog that is to small, I hope you have lots of space for your dog to run and play.