"If you found a fossil bone that was 3 inches long, would you know that the bone came from a small animal?"
Not necessarily. We are fairly large animals yet we have many bones in our bodies that are less than 3 in. long, and even really tiny bones in our ears. With no training and little experience, most people cannot identify small fossil bones without some expert assistance. But help is often available depending upon where you live. Many states and major cities (like New York) have natural history museums that employ paleontologists. The museum scientists there will likely help you identify fossils.