How do dragons work?

I don't really know very much about dragons. 

I do know that dragons are super important in the mythology of many world cultures. 

I also know that dragons are one of the most mainstream mythological creatures, along with unicorns and probably some other things I'm forgetting. If you asked a small child to draw a picture of a dragon, they'd probably be able to present you with a drawing of something resembling a dragon. I personally spent a lot of time as a child fantasizing about what my ideal pet dragon would be like.

A visual representation of my ideal childhood dragon that I found via a Google Images search for "purple dragon"

I recently painted a picture of a purple dragon setting everything on fire and it definitely looks like something a small child created. My mom thinks it's an expression of my frustrations but I'm pretty sure it's just a dragon. This isn't really relevant, I just wanted to talk about my dragon painting because I'm weirdly proud of it. 

Another dragon-related fun fact about my life is that I often find myself accidentally referring to my brother's Rhodesian Ridgeback dog as a Norwegian Ridgeback, which is a dragon from Harry Potter. Anyways.

How to Train Your Dragon is obviously a great movie, but here's a fun fact, it's actually based on a book! Wow! Incredible! Actually, it's an entire series, but I've only read the first one. The second one is on my list. 

How to Train Your Dragon by Cressida Cowell is kinda like the movie but also not really at all. 

One of the main differences I noticed about the dragons in the book version of the story is that they're a lot smaller. Hiccup's dragon Toothless at least is small enough to perch on his arm. I mean not all of them are that small, I guess according to the book the size varies pretty drastically between species, but definitely the ones the people in the village have are not huge. In the movie they're all flying around on their dragons like it's no big deal.

Also, in the book all of the children get to choose a dragon as a rite of passage and if they don't succeed in training it to hunt fish for their viking village, they're exiled, which seems sort of aggressive. In the movie, the vikings are all about slaying dragons, not training them to hunt. 

And then... well I won't tell you what happens, in case you haven't read the book and/or seen the movie, but essentially they're completely different stories.

Mostly, I'm just left with a lot of questions about what dragons are really like. You know, in real life. Real dragons. The ones that exist. 

...in reality.

How big are they? How do you train them? Do they really have their own language like Cressida Cowell asserts in her book? 

If anyone knows things about dragons, I'm really open to learning more about this topic, because like I said, I don't really know very much about dragons.