Can anyone tell me how to determine correct ear length and set?

If you had to choose between too long or too short, which would you take? Is one look faulted more than another?

Ears look out of proportion at times on puppies. As far as adults go, it's hard to say whether smaller or larger would be my pick. It depends on the shape of the head. I would almost rather have a larger ear than too small, but a large ear with a narrow muzzle is not good, either.

When pulled toward the face, the ear should reach to the lower corner of the eye. I'd much rather see an ear that is too large rather than too small.

I agree, I would rather have ears that are too large. Small ears give an odd look to the face. A friend kept a puppy from a litter last year hoping her ears would catch up. They never did and she looks really strange.
I did have a puppy buyer contact me once about their dog's ears. The dog was almost 3 and they decided they wanted a refund because his ears were too small! He was from my old field lines and was purchased as a pet. I said, No, but I will be glad to take the dog back if it bothers you that much. They declined.

"When pulled toward the face, the ear should reach to the lower corner of the eye."

I have had dogs that looked like their ears were smaller but always reached to the corner of the eye. Those that are claiming small ears, did the dogs ears not make it to the corner of the eye? I have never seen this. That would be small.

If a puppy, don't look for a year. If grown dog I had rather have larger than small ears. Small will also tend to look high set.

I have one that had HUGE ears as a puppy, then around 18 to 30 months his head finally stopped growing and I can now say they fit him fine and yes, they do reach the corner of his eye perfectly.