what breed of dog...?

Samoyed.

There are a lot of big, fluffy, white dogs. Here are some breeds. Google the breed name and see if pictures match what you're describing.

Samoyed

Great Pyrenees

Berger Blanc Suisse

American Eskimo Dog

Siberian Husky (with an all white coat)

How big?

Great Pyrenees? American Eskimo Dog? Maremma Sheepdog? Samoyed?

Do you mean A Great Pyrenees?

Like this one;>>http://s3.amazonaws.com/assets.prod.vetstreet.com/...

Great Pyrenees? There's several dogs that are white but this is the largest one.

Wow, way to narrow it down. Golden retrievers (despite the name they can often be white)? Chow Chow? Samoyed? a picture of the breed you mean would help, there are so many breeds which fit your description. My guess is Samoyed as the breed you're talking about.

Samoyeds.

Great Pyrenees, Standard Poodles, I have seen rare white Saint Bernards, and Leonbergers.

Are you talking about Great Pyrenees, or Spitz, or what?

Great pyrenees?