**Growth / size**

The main argument against mountain-sized dragons is the [square/cube law][1].  As an animal increases in size in its linear dimensions, but retains roughly the same proportions, its surface area and strength increases proportional to the square of its length, but its mass increases proportional to the cube of its length.  

To give an example of this, compare a mouse with an elephant.  A mouse has relatively slender legs, while an elephant has thick legs necessary to support its weight.  An insect is an even more extreme example of low size - they can have very thin legs compared to their bodies.  Taking the example of increased size to the logical extreme, if an animal was to continue growing, it would eventually become so heavy that it would no longer be strong enough to move itself, and/or its bones would break under its own weight.  The point at which this occurs depends on the strength and mass of the creature's body.  Obviously, if stronger, lighter materials than those found in terrestrial animals were in use, the animal could grow larger.

As the question specifies, using earth-like biology and conditions, the upper limit of size for a land animal would be on the order of tens of tons, certainly not more than a hundred tons.

**Flight**

The ability of a creature to fly is also dependent on the square/cube law.  Larger creatures just don't fly as well as smaller ones.  In an earthlike environment, the best fliers are no more than about a kilogram in weight, and flying creatures become progressively more clumsy as they get bigger.

There is a school of thought that a non-magical dragon could lighten its body with hydrogen gas trapped in large pockets within its body to the point that it could fly despite its apparent large size, however this would result in a dragon as fragile as any bird capable of flight.

Another school of thought is that *young* dragons fly in order to disperse themselves from their parents (since a carnivore weighing several tons would eat a whole lot), and lose their ability to fly as they grow.  An adult dragon may retain wings, but use them for display, not flight.

**Breathing fire**

As to breathing fire, this could be explained with the example of the bombardier beetle.  These beetles can produce a chemical reaction in their abdomens that is sufficiently energetic that the reaction byproducts are literally boiling, and it directs this from a nozzle at the end of its abdomen toward attackers.  Breathing fire is a logical extension of this sort of ability.

A dragon could produce and store a volatile high-energy fuel, or a liquid fuel with even more energy, and ignite it in many different ways, such as chemically-produced heat, or an electric arc.  By squirting the fuel out fast enough from a duct venting into the mouth as the dragon exhales, it need not ever come into contact with the burning fuel. (Think spraying the flammable gas from a spray can over a cigarette lighter - it doesn't melt the plastic nozzle.)  With a change in biology so that a dragon could precipitate metals such as aluminium or magnesium, a dragon might even be able to spit a liquid mixture akin to thermite that would spontaneously combust due to the presence of other reactants.

Or, as Anne McCafferey proposed in the Pern series, exposing certain rocks to acids produces spontaneously-flammable phosphine gas.

The main problem with fire-breathing is the amount of energy required to produce the reactants.  Whether the dragon itself or some symbiotic organism produces the reactant, it takes an input of at least as much energy to produce the fuel as the release of energy when the fuel combusts.  The best solution is if the energy comes from outside, as is the case with Anne McCafferey's Pernese dragons.

**Fireproof**

The point of "breathing" fire in the manner I have suggested means that for the most part, a dragon need not actually be *too* fireproof, since like a human fire-eater, the fire is not *inside* the dragon.  On the other hand, a certain degree of heat resistance *would* be useful for a creature that could make a mistake with its own fire - or get into a fight with another creature that also produces fire.

There are a number of adaptations that could help a creature that must maintain a relatively low body temperature deal with extremes of heat.

1.  Some organisms (known as extremophiles) are known to live and grow in temperatures below freezing and above boiling point, achieving this through specialized proteins.  Some desert living mammals (whose normal body temperature is 37°C) can survive a body temperature up to 50°C, whereas huamns start having problems with hyperthermia at 42°C or less.  It is not beyond the bounds of possibility that a dragon could have evolved proteins that allow it to survive much higher than normal body temperatures, perhaps well beyond 50°C, though most likely *not* over boiling point.

2.  Where parts of animals bodies experience extremes of temperature, counter-current heat exchange mechanisms in their blood vessels greatly reduces the flow of heat to or from the extremities exposed to those temperatures.  By selectively passing blood through countercurrent heat exchangers or not, peripherals can be maintained at a higher or lower temperature than the body core.

3.  Carbon (which is in relatively abundant supply in carbon-based organisms) in the form of nanotubes or sheets has very high thermal conductivity.  By strategically placement of such materials, the heating effect of point heat sources could be spread over a wider area and even transmitted rapidly deeper into the body, preventing burns by allowing an overall but lower increase in body temperature.  Such materials also have the added bonus of being very strong.  It is not beyond the bounds of possibility that a dragon may have evolved to be able to produce such substances.

4.  By maintaining a high body mass, an organism which uses water as a solute for its metabolism would require a great amount of heat to raise its body temperature significantly - water has amongst the highest heat capacities of any substance per unit mass.

5.  Creatures whose body temperatures rise higher than normal must eliminate heat, by radiation and/or evaporation.  Having a large pair of wings gives a dragon a large surface area through which it can radiate and evaporate excess heat, and its large mass gives it plenty of water that it can evaporate to shed the large amounts of heat that may be involved.

The combination of all these factors may result in a creature that, at its adult size, could be trapped in a burning wooden building for quite some time before it was particularly inconvenienced by excess heat, possibly on the order of some minutes to a quarter hour or so before its body temperature rose to a point that threatened its life.  If it escaped such a situation alive, it could then shed the excess heat quite quickly.

**Carnivorous**

A carnivorous lifestyle is the most likely thing about a dragon.  Carnivores obtain their energy far faster than herbivores, though they do have to work harder to catch their food.  Herbivores can't afford to stop chewing for long, but carnivores (such as cats) can have a relatively quick meal and then sleep the rest of the day.

  [1]: http://en.wikipedia.org/wiki/Square-cube_law