Dragons could very well exist somewhere, living quietly, but you've got to answer for a few of our given assumptions about dragons first.

- Bone structure. A creature that size would have to have a very different bone structure from any other creature we know of, or else it's skeleton would splinter under its own weight.

- Wings. This goes hand in hand with the previous statement. A bird's wings are, on average, larger than its body, sometimes twice as big or more. Birds have evolved to have hollow bones (maybe some other stuff too, I don't know enough about it) to cut back on their weight so they can fly. Not a problem for something that small. Consider the weight question brought up in before, and you realize that for it to be able to fly its wings would have to be HUGE, like four or five times its body size at the very least. Not to mention how the skeletal system of the wings could support it without them breaking or tearing off its body.

- Fire. Reign of fire had a fairly convincing explanation for this (at least to me, I'm not as smart as a lot of people on here, so I may be wrong) but I'd still like to see some sort of research on how this could work.

Of course, you could just chalk all that stuff up to magic and leave it at that, but that opens up a whole another can of worms on whether you believe that's possible or not (I personally think it is, but I doubt anyone has figured out how to utilize it yet).