The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
20. And on account of the two latter (i.e. going and returning) being connected with their Self (i.e. the agent), (the soul is of atomic size).
We admit that 'passing out' might possibly be attributed to the soul even if it does not move, viz. if that expression be taken to mean the soul's ceasing to be the ruler of the body, in consequence of the results of its former actions having become exhausted; just as somebody when ceasing to be the ruler of a village may be said to 'go out.' But the two latter activities, viz. going and returning, are not possible in the ease of something which does not move; for they are both connected with the own Self (of the agent), going (and coming back) being activities abiding in the agent 1. Now going and coming are possible for a being that is not of medium size, only if it is of atomic size. And as going and coming must be taken in their literal sense, we conclude that the passing out also means nothing but the soul's actual moving out of the body. For the soul cannot go and return without first having moved out of the body. Moreover certain parts of the body are mentioned as the points from which the soul starts in passing out, viz. in the following passage, 'Either from the eye or from the skull or from other places of the body (the Self passes out)' (Bri. Up. IV, 4, 2). Other passages mention that the embodied soul goes and comes within the body also; so, for instance, 'He taking with him those elements of light descends into the heart' Bri. Up. IV, 4, 1); 'Having assumed light he again goes to his place' (Bri. Up. IV, 3, 11).--Thereby the atomic size of the soul is established as well.
36:1 Going is known to be an activity inherent in the agent, from the fact of its producing effects inherent in him, such as his conjunction with--or disjunction from--other things.