A fun thing to do is to go onto AnimalPlanet.com and look up Dogs 101 videos and compare breeds. I know of quite a few medium to large sized dogs that are protective of their families yet sweet with even children. Larger isn't necessarily better. Some larger dogs just may be too much if it's going to be an inside dog and often times medium breeds are big enough in personality.
I'm honestly not a big fan of mastiffs. My brother has one right now and it's a HUMONGOUS dog at only a few months and even though he's being trained, he's difficult to deal with because even minor damage they do can be considerably bigger. They're nice enough, but hard to keep in the house. I would personally recommend a well built medium sized dog that's easier to train.
I've done a lot of research on various dog (and cat) breeds over the years (kind of as a hobby) and tend to be a rather random database on the temperaments of many breeds of dogs. I've personally had a lot of experience with Laboradors, Beagles, Weimaraners, Vizlsas, Jack Russell Terriers (or Parson Russells), Poodles (standard and minature), Pomeranians, Shih Tzus, English Mastiffs, Dachshunds, Airedale Terriers, Rottweilers, and varies mixed breeds. I also volunteer at an animal shelter and have trained and helped trained both dogs there, neighbor's dogs, and my own as a hobby. If you want to know anything about a particular breed, let me know.