The size of an atman could not be determined by any means physical or otherwise. More than that so many people world over did not believe in existence of atmans.
The most human beings could reach was God particle. Mankind no matter how much science evolves could never reach world of atman, consciousness within us.
Science demands proofs that are not forthcoming in field of spirituality, world of atmans.
As detailed in Bhagavad Gita and Upanishads (independent spiritual treatises) an atman is so minute in size that collectively all atmans souls in cosmos carried the volume of half a thumb.
Uncountable, do not know how many trillions and more atmans existed in whole cosmos. But the combined power of all atmans in cosmos in their pure form is what we called God Almighty, Brahman in all its glory. It is true that Brahman, God existed in all human beings in miniscule form as our atman, consciousness within.
As ordained by God, everything relating to atman soul can be realized only when human beings indulged in spirituality detailed in Bhagavad Gita to reach end of cosmic life, state of enlightenment in their lifetime.
It is primarily Upanishads that detail everything relating to an atman soul existing in human body. By diving deep into the bottomless pit of wisdom contained in Bhagavad Gita we can not only know everything relating to atman but also realize self, realize god within our lifetime.