Many gens are responsible for gens maybe able to add an extra 6 in (15 centimeters) to that average but beyond that we are likely to hit a ceiling. Regularly in order to produce people over 8 feet tall 2.44 meters these people would probably need to be a different shape not human shaped this is because of the square-cube law. As a shape grows taller it's volume increases at a greater rate take a look at this cubic if we make it ten time larger sure it is times long, tall, the area covered by spaces in one hundred times larger. And the volume the space within it in 1000 times larger. So 100 time area to support it self and 1000 time more are to support.