Here's a case of where my knowledge of physics and related sciences has proven inadequate, so I'm asking for help in answering the following question: assuming a world much like Earth in terms of gravity and air density, what is the largest plausible size for a flying creature that's not some paper-thin jellyfish-like thing? Now, assuming you're free to adjust gravity, air density, and other factors, is it at all reasonable to imagine any world where a human-sized being could ride a flying creature?
These questions are pure speculation. I'm asking only because I was thinking first about flying dragons and then, inevitably, about dragon riding, both of which seem impossible without magic doing the heavy lifting. But then I thought maybe I was being too hasty and there was some way to justify such things. So, what's the considered opinion of the readership of this blog? Yea or nay?