It really depends! I have 2 Shih Tzus that are sisters. One weighs 11 lbs. And the other one weighs 22 lbs. Big difference and they are from the same litter!
How Big Will My Shih Tzu Get? She Is 5 Pounds At 5 Months
She should be around 6 pounds, it depends how big she was at 3 motnhs then you double that. At 6 months they should be fully grown.