Found an interesting article about that topic: http://www.bbc.com/future/story/20150513-will-humans-keep-ge...
Looks like we are record holders in height already, let's see how far we can get!
Obviously, if a height increase/decrease trend continued for a while, one assumes genetic variations which aided this would be selected for but it seems that the change itself doesn't necessarily need genetic as such.
See height increases in Japan:
> The same reason why the average man was 5 inches shorter in 1750 than he is today.
This is more likely dietary changes than resource restriction—we may not have had industrial farming, but the average person wasn't starving either. But you need access to plentiful, high levels of protein to maximize your height advantage.
Which can be seen intuitively because non-human predators pick off the weakest and smallest of the pack.
Hence a resource restriction?
You can't be bigger than your container.
Btw, the show was fairly good, imo. Deep, and nearly hard SF.
Think of something like The Field Circus from Charles Stross's "Accelerando".
TLDR: Interstellar multi-generation spaceships are essentially genetic islands.
This, by the way, means that there really were dragons in sub-Roman parts of the Dark Ages world: Wales and Cornwall, for example, where draco-bearing cavalry (including King Arthur's, if there was a King Arthur) were fielded probably until the 600s or 700s.
But dinosaur bones driving myths of biblical humanoid giants? That's a much bigger stretch. Most dinosaur bones are pretty non-humanoid, and people who are generally a lot more familiar with human and animal skeletons than we are today wouldn't mistake one for the other. Also, while dragon mythology is mostly reasonable based on fossil evidence (except for the flying and fire-breathing... and gold hoarding) giant mythology is completely different. The biblical texts and non-biblical books like Enoch give them names and actions that are much more historical sounding than mythological. You don't get details like that from big leg bone fossils.
So, I don't buy dinosaur bones as the source of giant mythology.
(To an extent, dragons fall into the same category as unicorns, griffins, and phoenixes: not mythology so much as really bad zoology. All four of these species were believed to exist in the real world as contemporaries of medieval humanity, sometimes surprisingly nearby -- like griffins in the Caucasus.)
A related thought is that if there ever was any other intelligent species on Earth, noticeably different than humans, we wiped them out long ago.
If they have a different number of chromosomes it's likely.
(which I think illustrates the complexity more than it does anything else)