The aura is created by the interaction of all the energies and inter-dimensional forces that make up the human being. All of the energies and forces that are in and around you are meshed together and extend outwards from the body. The etheric usually extends about one inch but can extend further under certain circumstances, the astral body and chitta extend outwards further, usually between 2-4 feet and the light body can extend a very long way - even hundreds of miles.
Depending on the state of your subtle bodies and your conscious control of them your aura can be very open or quite closed. In fact the state of the aura can vary very widely. On some planets the inhabitants leave their auras wide open as there is no negativity and they can allow themselves to merge with each other and their environment. For life on Earth the ideal aura is fairly closed with a solid border to block negative energies. This is needed here because there is a huge amount of negativity floating around, mainly in the astral realms and it is best not to be absorbing this into your subtle bodies. Some people’s auras are quite closed due to a very solid ego structure which protects them from outside energies while others may have holes and be open due to gaps and a generally weak ego structure. If the structure has gaps or is weak then it is a good idea to take control of your aura and learn to seal it. If the ego structure is solid then the need for this is not as great unless you begin with spiritual work. At the beginning of spiritual work the ego structure must be dismantled. This can leave the aura open to external influences.
For this reason sealing the aura is a good idea for anybody doing spiritual work. A very effective technique is given in Past Life Regression