I wud suggest you first read the difference between a watch dog and a guard dog and then only proceed with your search further. You also need to see the living conditions before deciding on a dog breed. Apartment, house with big backyard or lawns, time available to train and take care of the pet, cost of feeding, etc.
Small breeds make excellent watch dogs.
Please also read article and other Q&A on this website....just recently someone bought a lab and wanted it to act as an adult at 5 months or so. Do ur research properly and you will be a happier person with a happy dog end of the day.