Bhagavad Gita Chapter 2 Sloka 23 (2.23)

ನೈನಂ ಛಿಂದಂತಿ ಶಸ್ತ್ರಾಣಿ ನೈನಂ ದಹತಿ ಪಾವಕಃ |
ನಚೈನಂ ಕ್ಲೇದಯಂತ್ಯಾಪೋ ನ ಶೋಷಯತಿ ಮಾರುತಃ ॥೨೩॥
nainaṃ chiṃdaṃti śastrāṇi nainaṃ dahati pāvakaḥ |
nacainaṃ kledayaṃtyāpo na śoṣayati mārutaḥ ||23||
Gist of the sloka:
The soul can never be cut [or destroyed] by weapons, fire cannot burn it, water cannot soak it and air cannot dry it.
Explanation:
For destroying anything there are only 4 means. Earth [metals from it, used for weapons], water [through decomposition], nor the fire [can burn it to ashes] or air [which can dry it].
Rishi’s estimate the soul size to be size of 1/10,000th of the smallest hair on horse tail.
Certain other philosophies try to explain the size of soul to be flexible occupying the entire body where it is resident. This cannot explain among other things how it can shrink in case it gets a body of a minute organism in its next life.
If one realizes that the soul is a bright light of knowledge, then like a bulb which though present in a corner of the room can illuminate the entire area, soul too resident near the heart can spread its energy through the body irrespective of its size [elephant, whale to amoeba] without having to keep re-sizing itself every time it changes body.
The great sages’ explanation of the size of the soul perfectly explains it. This is as per Vedas too.
The next question would be, is there nothing which can ever physically destroy the soul?