How much should a girl 12 years old supose to weigh when they are 5.2 tall?

The general healthy rule for girls is 100 lbs. at 5' and an additional 5 lbs. for every additional inch. Therefore, a girl who is 6' would most likely be healthy at 160 lbs. This is, of course, extremely overgeneralized; there is no 'right' number and it heavily depends on your body type and other personal factors.