A full grown female weighs typically 80-90 pounds. I would imagine a 6 month old female would weigh around 60-70. If you think your dog is underweight talk to your veternarian and if you don't have a vet you shouldn't have a dog....
I have a 6 mo male. He weighs 55 lbs. He seems like he's stopped growing but I'm with him everyday so it's hard to tell