I found a 6 quart slow-cooker at Target for $24.99. With 6 quarts, I could make some pretty big batches of stock; eh? I gave it a try.
To around 4 pounds of turkey neck bones I added onion, garlic, carrots, and celery, and 2 quarts of water. Yes, just 2 quarts. I cooked this around 21 hours on low. It yielded 2 quarts of stock; believe it or not.