Anything is possible.

<h3>Fireproofness</h3>
It can be easily imagined that scales could form in hardened way so they do not burn. 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.

<h3>Growth</h3>

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.

<h3>Wings</h3>

Wings are easy, it is not hard to imagine a reptile with wings. They would be controlled with muscles within the breast of the beast, likely any other winged creature. The wing's size would be different for different sizes of dragons.

<h3>Firebreathing</h3>

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. However, if the gas was somehow blown back down the dragons throat, it could be killed that way.