Try the dog breed selector at
www.selectsmart.com as well. You can also look up info on temperament, health problems, etc., at
www.dogbreedinfo.com.
Can you say more about what you're looking for besides size? Do you want a dog that is intelligent? Independent? Protective? Good with kids? Can you stand barking? How much room will they have to run? How much time will you be willing to devote to training and exercise?