It turns out there are a few factors preventing us from growing as tall as the dinosaurs. But the primary one is our environment. Gravity and the limited buoyancy of the air around us means that our bones would have to grow exponentially larger to support a human that towered 3-4m tall. And our organs — like the heart — would have to be considerably larger to pump enough blood to keep us alive. Of course if we chose a life under the sea, like blue whales have, the extra buoyancy there would easily let us grow into Guinness World Record-smashing behemoths.