Olaf Jansen, among others (the ancient Greeks), described how the inhabitants of the inner world were easily 12 to 15 feet tall. He also explains how they lived up to 1000 years. This can be easily explained as the inner world is sheltered from the harmful cosmic rays that bombard the outer surface and cause degradations of the genetic code and also make life more difficult and harsh. Before mankind reached the outer surface, they enjoyed an idyllic paradise in the inner world.
Navajo legends also explain that once the Ancient Ones descended into the inner realms to escape the cataclysms on the surface and when they reemerged many years later, they were much taller and stronger than before. This coincides perfectly with many ancient traditions and folklore from around the world.
Below are some images put together by a very talented photoshop artist of how the remains of our giant predecessors might look like (recommended by somethgblue in this video). They are only meant to stimulate the imagination and I am not suggesting they are authentic. Get a good idea of scale, however, and you will understand the magnitude of the claims surrounding our inner earth relatives!