Border Collies typically weight between 30-48 pounds--males tend to be larger. These are active, high energy, and very athletic dogs. With any dog its body condition score is more important than the weight (this is just a number). A dog with a good body condition has ribs that are easily palpated but not visual, dogs should have a waist when looking from above and a tummy tuck underneath. The hip bones and spine should be easily felt but should not jut. At your next visit with your veterinarian ask them to discuss your dog's body condition and ideal weight--this is different for every dog just like it is for people.
On average they are pretty light for their size. Female should be about 45 lbs and the males 50 to 60.