That’s a great question campbell!
Yes we do continue to get taller as a species, but there are some problems that may come with this. The taller you get, the stronger and bigger your heart will have to be in order to pump the blood higher – the same goes for your other organs – we won’t just be able to grow taller – lots of our organs will have to grow in proportion too!
There is also an issue of diet and sustenance – one reason why we have grown taller is due to improved living conditions – more and better food has effected us over a long period of time. Now though we may be getting too much of this – hence the rise in obesity. How will that effect our stature in the future? Will we continue to get taller or actually start getting smaller (and wider)?
We’ve only gotten taller in the relatively recent past, probably because out nutrition has gotten better, and perhaps because of a stronger “selection” preference for taller, rather than stronger/tougher perhaps, men. I think that the women are in control here …
I don’t think that this will continue until the average height is that of “giants” because you start to run into other physiological problems as you increase height. For example, as we get bigger, the distance between the heart and heart increases, which is real problem for getting blood to the brain – giraffes cope only by having some complex plumbing and small brains. Getting the blood back from the feet is more difficult too.
Also, there is a problem with heat. As animals get bigger, more surface area doesn’t change as much as their volume, so it is harder to get rid of waste heat. So, ignoring other factors, “global warming” (assuming it is happening) might cause positive selection for smaller people, as the big ones die off from over-heating!