How does an anesthesiologist know how much anesthesia to give?

Many things: An anesthesiologist will review a patient's medical history, any adverse medical conditions, and will base anesthetic choices and amounts on any medical conditions and a person's size and weight. The anesthesia may be titrated or adjusted during surgery, depending on the expected length of the operation as well.

Multiple clues: Intravenous medications are often started based upon weight. There are also clues based upon vital signs such as pule rate, blood pressure, muscle tone, etc. Experience also plays a huge role.

Weight and history: The amount of anesthetic drugs used to put you to sleep is judged by your weight and your medical history. Once you are asleep you are usually kept in this state by breathing an anesthetic gas. The anesthesiologist will be monitoring your vital signs during the case to make sure you don't get too much anesthesia. This can be seen by a drop in blood pressure, change in heart rate, or other factors.