Your horse's smart halter should fit comfortably, but not so loose that it is easily removed. The noseband should sit about 1" below the bony protrusion of the cheekbone, but not so low that it sits on the delicate structure of the nasal bone. The noseband should be loose enough to allow your horse to comfortably graze and chew their grain and hay. The buckles and hardware should sit well away from their eye.
So, you have a pregnant mare and aren't sure when to order your smart halter so that you can be alerted to foaling? (The short answer: 2 - 4 weeks before you need to be alerted.)