A good tool is this dog breed selector that asks you all kinds of questions about characteristics you're looking for in a dog, as well as what kind of training/exercise/grooming you're willing to put into having a dog:
<a href="http://www.selectsmart.com/beta_dog/">
http://www.selectsmart.com/beta_dog/</a>
You might also look at <a href="http://www.dogbreedinfo.com">
www.dogbreedinfo.com</a> for information on specific breeds, including known health issues, breed characteristics, etc.
I would NOT NOT NOT get an aggressive breed dog from a rescue org unless you know the dog's history with 100% confidence - you do NOT want to be adopting a presia canario or a rotweiler or such with a history of abuse or aggression. It can make them unpredictable, and you need to be an extremely dominant, consistent and strict trainer. Raising a cocker doesn't prepare you for that.
You might want to look into a dog that's a good "watchdog" but not necessarily a dog breed with a tendency to aggression. An australian cattle dog (or blue heeler) is a compact, energetic breed that is easy to train and EXTREMELY loyal. They're unlikely to attack unless you are being attacked, but will bark and protect you unless you give them indication that a visitor is okay.
Above all, get a dog that you are confident you can command and control, and that has an activity level you are willing to commit to. A smart, trainable dog is likely to require a good deal of attention and training, and that goes double for ANY dog breed that has inbred aggression.