I like my science fiction and fantasy to be pretty grounded in reality with a bunch of well-delineated differences. My suspension of disbelief breaks down pretty quickly if, for example, a setting contains magic that would totally warp of wreck the society and economy, or if basic physics like conservation of momentum is casually disregarded.
On the human side, tropes like dictatorships where the dictator has no defined power base beyond "everybody obeys him, because he is in charge", or knights that are genuinely all brave and noble, are no better.
Obviously, if the work is meant to be just a bit of fun, I care rather less. But if it's meant to be an even vaguely serious work - or a role-playing setting I'm playing in - I like my world to make some sense internally. With roleplaying this is especially important: I like coming up with novel solutions beyond "frontal attack, killing everything", and it helps when there's a consistent physics, economics and society to support these solutions: can I hire mercenaries? Smoke them out? Hire a magician to set them all on fire, or put them to sleep, or to put a glamour on us so we'll be considered friends? Can I obtain some fake ID? Redirect a river? Pour a bag of scorpions down the chimney?
So it really helps if the exceptions - the magic or science - are well-embedded in the setting. If there's a teleporter, can I use it to teleport people into the sun? The sun into people? If I have an airship, can I use it for aerial bombardment by dropping large stones from a great height? If I can Charm Person, can I just go around and keep on trying to charm people into giving me all their stuff? The answer to all of those can be no, but it should stem from the way these things actually work in-universe rather than just being a "no, you can't do that". If I keep on bumping up against invisible walls when trying to come up with interesting solutions, this puts a real damper on my degree of interest.
To dragons, then. Obviously, dragons can be regarded as magical creatures, in which case they can do whatever they want: weigh 500 tons and fly despite having wings that are proportioned for a medium-sized bird, etc.
But let's see how much of a dragon we can make while not explicitly violating physics or biology or chemistry. We want a giant fire-breathing reptile. Well, the largest flying animal we know of is Quetzalcoatlus. Demonic or not, this pterosaur had a ten-meter wingspan and may have weighed as much as 250 kg. It's not clear to what degree Quetzalcoatlus was able to do powered flight - it may have been extremely reliant on thermal soaring - but it's not impossible that something like it would be capable of dramatically swooping over a medieval town, striking terror into the hearts of the peasantry.
To the fire-breathing then. The big problem here is that actual fire inside your mouth is not a safe thing: human fire-breathers can "breathe fire" by spitting out flammable substances and igniting them once they are somewhat safely outside their mouths. A dragon could do something quite similar: there are plenty of substances that are inert until they come into contact with oxygen, at which point they ignite. Further, it would be possible for this substance to only ignite when aerosolized, and when the concentration of particles is low enough, or to perhaps have a short delay before ignition. Dragons would hence actually spit out a jet of flammable liquid that would turn into a fine enough mist to ignite just past the snout.
While impressive, the damage done by this fire might be somewhat limited, and the dragon could certainly not keep breathing fire all day - a few solid gusts of fire would be enough to exhaust its supplies for a while. Dragon fire doesn't make a lot of sense as a weapon for hunting, then, but it would serve well enough as a threat display - or as a sexual display. This could also be drafted to explain why such an already formidable creature would develop such a redundant and metabolically expensive feature: dragon's breath is like antlers on stags or horns on sheep: not as much practical as impressive, and a kind of conspicuous consumption. Big antlers, a strong flame: both show that this animal is strong and healthy and capable of wasting its energy on such non-essentials.
There we have it, then: a quarter-ton dragon with a ten-meter wingspan that breathes fire when it wants to impress: as a threat or as a signal of fitness. Not quite as fancy as a giant magical dragon, but something about the size and shape of it makes it quite real, like something you might actually encounter - and then run away and hide very fast.