It honestly depends on your genetics. If both your mom and dad are tall, you are likely to grow faster before puberty and become taller than normal studies say. Your weight could also vary among your genetics, and also how much you eat, get activity, etc.
But, if you would like the details of what the studies say, here they are:
Average height for 12 year old male: 58 inches (4'11).
Average weight for a 12 year old male: 105lbs (52kg).
Average height for a 12 year old female: 66 inches (5'0).
Average weight for a 12 year old female: 105lbs (52kg).
