Anything is possible.
EDIT INFO: Ok, after a bit of research I'm changing some of my answer. I'll leave the original parts underneath the new parts, if some part was majorly edited. Most of the new information came from this Discovery Magazine article.
Fireproofness
Scales would be made out of bone. They would be made out of practically solid compact bone, unlike the ones in our body. This would give them the strength needed, without adding an unreasonable amount of weight. These scales also would not burn, unlike normal reptile scales. I think a dragon that could withstand very high temperatures (Ex. swimming in lava), isn't possible under earth-like biology. Most standard internal organs need a pretty constant internal temperature. But with a well insulated skin (lots of fat), and fire-proof scales, a dragon could probably withstand what fire he causes.
Growth
Your dragons size is very interesting, growing all the way from a small creature to as big as mountain. At first this may seem fairly fantastical, but it isn't too bad. The dinosaurs did practically the same thing. So I think we can say that this is possible, and has even be seen in real earth biology.
Flight
Wings are easy, it is not hard to imagine a reptile with wings. They even existed in the form of Pterosaur. They would be controlled with muscles within the breast of the beast, likely any other winged creature.
The main problem of dragon flight is that they are just too big. To get enough lift for the massive creatures would make the wings too unwieldy. But it is possible that they could still fly. The dragon's bones would be hollow, like both birds and dinosaurs were. but instead of being filled with air, their bones would be filled with lighter than air gases such as hydrogen, helium, or, most likely, methane, which is a byproduct of flatulence. The dragon's body would also be filled with large sacs filed with these gases as well. This would also make dragons more agile, because they would be easily light on their feet. This would also mean they could look pretty much like anything, because aerodynamics is no longer in effect.
Firebreathing
This idea specifically comes from the discovery magazine article listed above. 160
Dragons would have two sacs with holes that enter to their mouths. Inside one sack live organisms (like yeast) that produce ethanol. Inside the other lives bacteria that produce sulfuric acid. These two gases are allowed to mix inside the dragon's mouth, just before it breathes fire. The chemical that is produced when these two mix is Diethyl ether, which is highly flammable. The tiniest amount of friction can light it (spilling a beaker on a table is capable of lighting it). The dragon would have to not inhale while this was going on (this would of course be an involuntary reflex).
As the scientist who was interviewed in the article above points out, we don't always know how something evolved. But similarly complex defense systems have evolved on earth (the bombardier beetle is an example). A guess as how the sacs are formed is given below.
The original purpose of the two "sacs" was a very simple, an ineffective, breathing mechanism. These sacs were criss-crossed with blood vessels and bones, leftover from gills, and used for the first mechanisms of breathing. As a better breathing system (lungs) developed, the sacs fell out of use, and bacteria and yeast took over them. These then began producing the above substances to create fire.
EDIT: The next too sections are outdated but kinda fun, so I left them here. This is another technical possibility of fire breathing
This idea comes from the How to Train Your Dragon books. All dragons could have two holes in there mouth, these holes would excrete a highly flammable gas during exhaling, this gas is then breathed out of the mouth. At the edge of the mouth, there would be an instrument (likely two special teeth) that when rubbed together would produce a spark. When the spark hit the gas, it would light, creating a plume of fire. This gas would be hydrogen, made from splitting the water molecules made from splitting glucose. This would not have a significant impact on the dragons energy levels, because it would use the energy made from the glucose to split the water. The carbon-dioxide made from the split would be exhaled as normal, and the oxygen would be released with the hydrogen to increase the flammability of it. (One hole releases hydrogen, the other hole releases oxygen). Both gases are stored in special sacks behind the holes.
To protect against the mouth being burned, the front part of the inside of the mouth would also be covered in scales. No essential organs (not even the nose) would be housed near the front of the mouth. 671