Well this can be seen as 2 ways, the broad one and the less broad one:
1- at no point in history have your dragons ever obeyed biology or physics.
This scenario is the easiest to answer: unknown. If your dragons have never obeyed biology or physics (especially regarding the square cube law), then for all we know prehistoric dragons could've been as small as a bumblebee bat to as large as the Everest. Nothing has ever limited their growth and thus its impossible to determine and the option is up to you.
2-at some point in history they were still bound by the natural laws of our universe
Disclaimer: from this point on, I'll assume your modern dragons have a body plan more similar to that of a wyvern (4 limbs total, with 2 front limbs turned into wings), since you used the model of Smaug from the Hobbit trilogy of movies.
This is scenario seems to be the one you're trying to follow since you went as far as to research about prehistoric animals to see if there was a pattern, and normally there still wouldn't be a clear pattern, except for a key trait: flight
Flight is an ability that has truly evolved only about 4 times, as far as we're aware, 3 of them occurring in vertebrates. Why is it important? Because of 2 little things regarding its evolution in these groups specifically:
flight is no walk on the beach, while even animals highly adapted for living on land can still swim (see horses and ostriches), air is a lot less denser, and even the act of falling with style (aka gliding) usually requires a fair number of adaptations.
the bigger you are, the harder it is to fly.
Couple these 2 bits of information with the data we have from these 3 groups of vertebrates, more specifically pterosaurs, and we have a pretty good idea of why exactly they were more lively to have started out smaller than bigger:
1- essentially all examples of early flying vertebrates he know of were not that big. Certainly they weren't something absurdly small, but one thing we know is that they didn't include anything the size of dakotaraptor. This is easily explainable by the fact, according to the square cube law, means not only that you're lighter, but that you're stronger in proportion to an animal twice as large. This mix of reduced weight and proportionally stronger muscles means it'll be a lot easier for a smaller animal to ever begin exploring the niche of flying predator that it would be for something much larger.
2- once you got them wings developed, keeping them functional as you scale up becomes harder. For this bit, we'll look at the best example we have: the azhdarchid family. This group of pterosaurs is the one that houses not only the biggest pterosaurs to ever live, but the biggest animals that ever took it to the skies (on its own, at least). When looking at these pterosaurs and their adaptations, one thing becomes clearly obvious: they were extremely adapted for flight, from the lack of tails to incredibly hollow bones, these creatures were adapted for flight to an extreme, and chances are that these adaptations could only really come from a smaller animal that needed them to take place in order to grow without sacrificing the ability to fly.
And don't think these extreme adaptations meant they couldn't be proper predators, since examples of particularly robust azhdarchids like hatzegopteryx, which is a species of pterosaurs that most likely ruled Hatzeg Island as the biggest predators around, exploiting the insular dwarfism and how it resulted in an environment filled with smaller dinosaurs it could likely hunt.
So tl;dr: if your dragons have never respect the square cube law or any other principle that'd allow them to be larger, heavier and more powerful than any sauropod without giving up on flight, we can't be sure, simply because no laws are binding how big they could've been before magically deciding they wanted to be able to fly (and yes I'm assuming flight necessarily came after unless everything in your planet could fly at some point). BUT if their ancestors were indeed bound by natural laws at some point, they would have most likely started out as smaller aerial predators, later deciding they'd let no laws bind them and challenge all logic to become their current size and outlive any turtle by millenia.
Also, unless we have some seriously big megafauna rich in nutrients and energy your dragons will have problems with getting their daily requirements to live. Being that absurdly big and powerful while still being able to fly and all of that without sacrificing a powerful brain would probably mean they'd make a hummingbird's normal state of being at the edge of survival (they can starve in about 5 hours at their normal metabolic rate if they go that long without eating) look like it's not that bad.
Edit on something I forgot to mention: "being cold blooded" and "can live in almost any environment" are pretty much mutually exclusive, especially if you also want them to be flying vertebrates. While their size would normally make dispersing heat a big problem, when we look at nature we can see that:
1- not flying vertebrate on earth was ever truly cold blooded. Even pterosaurs were at least in the path towards endothermy, especially since flight requires a constantly high metabolism more easily found in warm blooded animals.
2-cold blooded animals, being highly dependent on environmental heat, don't really do well in polar/very cold regions. If you want dragons that can live in Antarctica or in mountain tops, you'll need them to be at least mesotherms (we suspect not even sauropods were truly cold blooded given how fast some of them grew up).