The commentary on this annotated version of the Bhagavad-gita that I'm reading talks about the soul:
This soul is described as one ten-thousandth part of the upper portion of the hair point in size. The Śvetāśvatara Upaniṣad (5.9) confirms this:"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." Similarly the same version is stated:
śatāḿśaḥ sādṛśātmakaḥ
[Cc. Madya 19.140]
"There are innumerable particles of spiritual atoms, which are measured as one ten-thousandth of the upper portion of the hair."
Therefore, the individual particle of spirit soul is a spiritual atom smaller than the material atoms, and such atoms are innumerable.
I was curious if this size was smaller than our present-day ability to detect.
Let's say that a human hair is between 17-181µm. (1.7×10-6 to 1.81×10-5 meters). This commentary says to divide that by 1×104, which gives 1.7×10-10 to 1.81×10-9 meters diameter for a soul particle. A hydrogen atom might be 1×10-10 m to 5×10-10 m in diameter, which would give a soul particle anywhere from 18 times the diameter of a hydrogen atom to about 1/3 the diameter. I guess it's still vaguely conceivable that the smallest possible size of soul particle may have gone undetected.
