As far as I know, an early neuter doesn't affect the dog's eventual size. I think that's pretty much an effect of heredity (given healthy foods, etc). I know here in California, if you were to adopt an animal out of the pound, it would be neutered before you could bring him/her home. We've adopted all of our cats from the pound and they are all huge!
I know that many breeders feel that a dog will not get as muscular if neutered too early, but I think that needs to be weighed against all the other aspects of full maturation (humping, marking territory, more aggressive behavior). In the Newf world, there is a discussion about hypothyrodism in neutering/spaying an older dog. There seems to be some connection between the two.
All in all, I think neutering any time after about 7 - 8 months is fine...earlier if necessary!!
This is of course, my humble opinion!!!