The SIZE PROCESS® model is based on sound psychological findings and combines approaches from depth and humanistic psychology as well as classical and systemic transactional analysis. It also integrates bioenergetics according to Alexander Lowen and Ron Kurtz as well as systemic approaches and the latest findings in brain research.

Between 1999 and 2004, the model was developed by Hannes Sieber (Germany) and Fritz Zehetner (Austria). The name SIZE is made up of the first letters of their surnames: SI (Sieber) and ZE (Zehetner).
Since 2004, the model has been continued by Fritz and Gabriele Zehetner and is internationally protected as a trademark. In cooperation with experts, Fritz Zehetner has continuously developed the SIZE PROCESS® model to this day.

In contrast to the classic typologies of the 60s and 70s, Fritz Zehetner considers people to be living ego systems. He sees personality as an interplay of six personality styles that are in constant interaction with each other and with the environment. Each person makes decisions through inner self-organization. Zehetner emphasizes that the individual personality styles cannot be considered in isolation, but only in their systemic dynamics and their connections to each other.

The SIZE PROCESS® model assumes that people construct their own reality because they do not have direct access to an objective reality. Each person thus creates his or her own, individual view of the world.