At birth Chihuahua puppies will weigh a mere 2.5 to 5.5 ounces. Within just 2 weeks they can double in size. By 4 weeks weight can vary from 7 ounces to 1.3 pounds and within 8 weeks they can weigh from 11 ounces to 2 pounds. At 3 months they'll weigh between 1 to 3 pounds.