Little dogs often have big personalities, sometimes so big that they become aggressive in everyday situations. "Small dog syndrome" is a term for this type of behavior. Like many "bad" dog behaviors, this type of behavior in small dogs is often formed by the owners. Small dog syndrome hatches when a little dog is allowed to get away with behaviors that are found annoying or bad in bigger dogs; it's also caused by an owner constantly picking up the dog.
Ignore the dog. Pull up your arms to your chest, look above the dog and don't make any noises toward the dog. This doesn't mean ignoring that you have a dog who's being bad. On the contrary, ignoring this behavior by not giving the dog any attention, even verbal scolding, denies her what she's after.
Calm yourself and don't show behaviors the dog may think are aggressive. When an aggressive dog comes at you, even a small dog who's not intimidating, do not allow yourself to get worked up. Stand still, make no eye contact and ignore the dog's advances. Don't back away or give the dog an indication you are scared.
Redirect your little dog's aggressive behaviors by counter-conditioning. For example, if she's barking at the doorbell, train her to go to her crate or another specific spot when the bell rings.
Respect the dog and let her investigate you if you're visiting her home. Don't touch her, talk to her or make eye contact with her until she's comfortable with you. This lets her come meet you on her own terms without feeling like she's being pushed into a situation. Her owner should not pick her up during this time or during any of these behaviors.