The most common breaker size in a residential electrical panel is 15 amps. This size is typically used for lighting circuits and smaller appliances that draw relatively low amounts of power. However, it's important to note that breaker sizes can vary depending on the specific electrical system and the requirements of the building. Other common breaker sizes in residential panels include 20 amps, which is often used for general-purpose outlets, and 30 amps, which is commonly used for larger appliances like dryers and water heaters. It's always recommended to consult a qualified electrician or refer to local electrical codes for accurate information regarding breaker sizes in your specific area.