The short answer is, "Pretty damn..."
Dragons do most of their growing in dragon form. The more time they spend in human form over the course of their lives, the slower they will grow. Older dragons (5000+ years or so) are much bigger at that age than younger dragons (3000 years or less) will be at age 5000 because the younger dragons must spend more time in human form, thus stunting their growth. There were once far fewer humans on Earth, and those that there were didn't have much in the way of technology. Even modern weapons have a hard time damaging dragons. Stone spears and rocks are basically hopeless. Also, primitive humans were kind of stupid: they assumed everything they saw was magical and should be avoided if it was too big to eat. Many of them treated dragons as gods. A dragon could afford to spend the majority of his time in dragon form, thus allowing him to grow. Those days are passed, and while dragons are very powerful, they have to hide out among humans. This stunts their growth a bit.
On average, a hatchling is about the size of a Ford Taurus. They grow rapidly in their first 100 years, and as such are required to spend more time in dragon form than most older dragons. This is the dragon equivalent to puberty. Dragons don't have an infancy, since they are hatched with intelligence that exceeds most humans. They also posses some of the knowledge of their mother, especially things that happened while the egg was still inside.
As far as how big a dragon can get, I can't really say without giving away part of the books. As you've probably guessed from reading the excerpt, Veles Fraise is very old. In dragon form, he is GIGANTIC. Think aircraft-carrier size. The odds of another dragon ever being able to get that big are slim, since Veles was able to spend most of his life in dragon form, without having to hide among humans. This is part of why he hates humans as much as he does. Garrett Terago, in dragon form, is large enough that Meg can lie comfortably on his snout or fit in his hand. Benjamin Ardeth is a couple-thousand years older, and therefore bigger by about half, but Ardeth is old enough that part of his life was spent while humans didn't dominate the planet. So Garrett Terago will probably not grow at the same rate.
Some dragons enhance their shape-changing ability to allow them to assume different forms. Another character introduced in the first book, Aoni'a, is able to take any human form she wishes. She can even change her gender--and does. However, honing this kind of ability limits her growth, so Aoni'a is a bit smaller than other dragons her age, like Garrett Terago. She is therefore not as powerful in dragon form, though far from helpless. In subsequent books, a dragoness who is capable of disguising herself as animals is introduced. This requires a much greater strain on her growth, and as such she is very small for a dragon her age (she's fairly young) and therefore not as powerful.