It would appear that there is an upper limit on human height. Factors such as gravity, buoyancy, diet, genetics and our surrounding environment all play a part in keeping us from eating plants off the tops of trees.
More importantly, the human body is presented with serious challenges the further it grows outside the bounds of what is considered “normal” or average height. For example, the heart would have to work overtime to pump the blood around the body. Similarly, our bones would have to grow to abnormal sizes to support the weight and frame of gigantic humans.
At the moment, there is no evolutionary reason for us to be 10 or even 12 feet tall. However, if our environment were to change drastically, for example if we lived on Mars, then we would likely see significant differences in human anatomy.
Of course, creatures of all sizes are still able to do amazing things in spite of their vertical limitations. In that sense, size doesn’t matter. Yao Ming might tower above others on the court, but a dog has a a sense of sound and smell that extends much further than most other creatures. Maybe that’s why Air Bud was so damn good at basketball.