The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
19. (On account of the scriptural declarations) of (the soul's) passing out, going and returning, (the soul is of atomic size).
We now have to consider of what size the soul is, whether of atomic size or of a medium size, or of great (infinite) size.--But, it has been shown above that the soul is not a product and that eternal intelligence constitutes its nature, whence it follows that it is identical with the highest Self. Now the infinity of the highest Self is clearly stated in scripture; what need then is there of a discussion of the soul's size?--True, we reply; but certain scriptural passages which speak of the soul's passing out, going and returning, establish the primâ facie view that the soul is of limited size, and moreover in some places scripture expressly declares it to be of atomic size. The present discussion is therefore begun for the purpose of clearing up this doubtful point.
The pûrvapakshin maintains that, on account of its being said to pass out, go and return, the soul must be held to be of limited, atomic size. Its passing out is mentioned (Kau. Up. III, 3), 'And when he passes out of this body he passes out together with all these;' its going (Kau. Up. I, 3), 'All who depart from this world go to the moon;' its returning (Bri. Up. IV, 4, 6), 'From that world he returns again to this world of action.' From these statements as to the soul's passing out, going and returning it follows that it is of limited size. For motion is impossible in the case of an all-pervading being. And a limited size being once admitted, we have to conclude more especially that the size is atomic, since the hypothesis
of the soul being of the same size as the body has already been refuted in our examination of the Ârhata-system.