Hi,
dogs are pack animals, they never ever live alone. So we have to condition them to living alone. If a wild dog is sperated from its pack it will call for the pack (howl,bark) and if that fails search franticly and then find a safe place to wait and listen. A wild dog will go under a tree root, in a small cave, under a bush etc. Out of harms way.
Similiarities can be seen in the way an unconditioned dog behaves when we leave it. It will howl, bark, run around looking for us and then if we are lucky when all elese fails it will get behind the sofa or under the table and hide.
A cage is really a portable hiding place for a dog. But if we make it a nice protable hiding place then that is better. I always say that anything a dog does in its cage is not a problem. So I never buy a £50 M&S dog bed for in there because I don't want to get wound up when it rips it up.
I have an adult cocker bitch that spends two thirds of her day in a 3x2x2 ft cage. I would suggest a lab goes one or two sizes bigger. The dog should just be able to lay on its side with the nose and its arse just short of either end.
Terriers are different. Terriers like even smaller spaces. I used to have a Welsh Terrier who had a wooden box reccomended by the breeder that was little more than a small rabbit hutch he used to squash himself into but he loved it.
The main rule is with cages that if you cannot show your dog 100% attention then put it in the cage. That way it does not get used only at night or when you go out. The cage becomes so normal the dog feels safe and secure in there.
If you foolw that your dog will be fine. If you have to leave your dog in there all day, makes sure it is well watered and has stuff to chew etc and if it messes or is sick in there, then you have to take the blame for that, not your dog.
Ian