The size of the soul is one ten - thousandth part of the tip of the hair. It means that if we take the tip of the hair and cut it into one ten - thousand parts then that extremely tiny part is the actual size of the soul.
Svetasvatara Upanisad (5.9) confirms the size of the soul:
balagra – sata – bhagasya
satadha kalpitasya ca
bhago jivah sa vijneyah
sa canantyaya kalpate
“When the upper point of a hair is divided into one hundred parts and again each of such parts is further divided into one hundred parts, each such part is the measurement of the dimension of the spirit soul.”
Although the soul is extremely tiny but it is extraordinarily powerful. The soul is situated within the region of the heart and is the source and cause of consciousness and all the energies. Whatever the pains and pleasures we experience is due to the presence of the soul in the body. When the soul leaves the body, the consciousness goes away and the body falls dead. Mundaka Upanisad (3.1.9) explains that the soul floats in five kinds of air (prana, apana, vyana,samana and udana). Although the soul is very pure but due to material contamination it is not able to exhibit its spiritual qualities. So, the soul has to be purified from these five types of material airs so that it can exhibit its original spiritual nature.