When you go to a shelter there will absolutely be large dogs and small dogs of all temperaments, some in kennels and some free-range if they have the space. That being said, as someone else commented, perhaps try volunteering at a shelter! You can do good work and get lots of exposure to all types of dogs, and learn from experienced people how to get a read on dogs and how to handle different situations. Also, shelter dogs tend to act a little differently when they finally get a nice home environment — their personalities really come through when they get some love and stability.