Common pests found around homes, mice and rats are both potentially dangerous to people and their property. Certain differences can help homeowners tell these two rodents apart: A mouse is typically 5 to 7 inches long and weighs around half an ounce. A rat is larger, up to 18 inches in length including the tail, and can weigh up to one pound. At times, it is possible to confuse a juvenile rat with a mouse. However, young rats have much larger heads and feet than the typical house mouse.