This has been discussed repeatedly in these forums.
If you're coming from a working border collie perspective, which these Boards are, there really is no standard for size or appearance or anything else except working ability.
A 25 lb. dog who works well is perfectly acceptable and desirable, as is a 60 lb. dog. There's a huge range among working border collies, much more than will be found among so-called border collies bred for conformation showing or even other dog sports.
That said, I think you're probably close in estimating an "average" for females to be around 30-35 lbs, and males around 35-40 lbs. But there are so many exceptions to those averages as to make them pretty loose at best.
BTW, there's a border collie rescue that routinely cites 35 lb. dogs of either sex as being "petite." It makes me twitch, it's so ill-infomed.