The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
22. And also on account of direct statement, and of inference.
The soul is of atomic size for that reason also that scripture contains a direct statement to that effect, 'By
thought is to be known that atomic Self into which breath has entered fivefold' (Mu. Up. III, 1, 9). That the Self spoken of there as atomic is the living Self, i.e. the individual soul, we see from its connexion with breath.--Inference also favours the conclusion that the soul is of atomic size; i.e. we infer that from such passages as 'That living soul is to be known as part of the hundredth part of the point of a hair divided a hundred times' (Sve. Up. V, 9), and, 'That lower one also is seen small even like the point of a goad.'--But, an objection may here be raised, if the soul is assumed to be of atomic size, and therefore to occupy one point of the body only, the fact of sensation extending over the whole body would appear contrary to reason. And yet it is a matter of experience that men bathing in the Ganges or in a pond experience the sensation of cold over their whole bodies, and again that in summer people feel hot all over the body.--To this objection the following Sûtra replies.