If they are about the same size, cubs generally end up around the size of their mother and males generally end up closer to the size of the male father. If the dogs are of different sizes, it's almost certain that your dog will find itself in between. With a purebred, it's usually easy to know what you're getting into. However, predicting the adult size of a mongrel puppy can be tricky, especially if you don't know the size of the parents.
It's a reliable fact that most dogs don't outgrow their older parent. Yes, in general a puppy will be about the size of the parents. They can grow, but that's more common with mixed races than with pure races. Your puppy will probably reach their final weight around 10 to 12 months, but around 6 you can get a pretty accurate prediction of how much the puppy will grow.
If both parents pass on the recessive gene, the offspring will be liver-colored, but the most likely outcome is a litter of black puppies. Of course, this can be the simple case of genetics: sometimes two white canines can produce black puppies if they both carry the dominant “black gene”. Regardless of the method you use to predict (or guess), remember that sometimes even two small dogs can produce a puppy that is larger than normal due to family history. If you know who the father is, you'll have a better chance of estimating the size range of puppies based on the adult size of mom and dad.