Who is a siddha ? A siddha is one who has attained siddhi, i.e. "power, prowess, strength, ability", then a special kind of psychic and supernatural, miraculous, occult power. There are eight kinds of super natural powers called as "Ashtama Siddhis"
A classical definition of siddhas is given by Siddhar Thirumoolar: "Those who live in yoga and see the divine light and power through yoga are the siddhars (Tamil Chittar)"
Power of becoming the size of an atom and entering the smallest beings.
Have the creative power of God and control over the Sun, the moon and the elements and
Capacity to be quite light though big in size.
Capacity to weigh heavy, though seemingly small size.
Capacity to enter all the worlds from Brahma Loga to the neither world. It is the power of attaining everything desired.
Power of disembodying and entering into other bodies (metempsychosis) and going to heaven and enjoying what everyone aspires for, simply from where he stays.
Power of becoming mighty and co-extensive with the universe. The power of increasing one's size without limit.
Power of control over King and Gods. The power of changing the course of nature and assuming any form.