The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
24. On account of the term, (viz. the term 'lord' applied to it) the (person) measured (by a thumb) (is the highest Lord).
We read (Ka. Up. II, 4, 12), 'The person of the size of a thumb stands in the middle of the Self,' &c., and (II, 4, 13), 'That person, of the size of a thumb, is like a light without smoke, lord of the past and of the future, he is the same to-day and to-morrow. This is that.'--The question here arises whether the person of the size of a thumb mentioned in the text is the cognitional (individual) Self or the highest Self.
The pûrvapakshin maintains that on account of the declaration of the person's size the cognitional Self is meant. For to the highest Self which is of infinite length and breadth Scripture would not ascribe the measure of a span; of the cognitional Self, on the other hand, which is connected with limiting adjuncts, extension of the size of a span may, by means of some fictitious assumption, be predicated. Smriti also confirms this, 'Then Yama drew forth, by force, from the body of Satyavat the person of the size of a thumb tied to Yama's noose and helpless' (Mahâbh. III, 16763). For as Yama could not pull out by force the highest Self, the passage is clearly seen to refer to the transmigrating (individual soul) of the size of a thumb, and we thence infer that the same Self is meant in the Vedic passage under discussion.
To this we reply that the person a thumb long can only
be the highest Lord.--Why?--On account of the term 'lord of the past and of the future.' For none but the highest Lord is the absolute ruler of the past and the future.--Moreover, the clause 'this is that' connects the passage with that which had been enquired about, and therefore forms the topic of discussion. And what had been enquired about is Brahman, 'That which thou seest as neither this nor that, as neither effect nor cause, as neither past nor future, tell me that' (I, 2, l4).--'On account of the term,' i.e. on account of the direct statement, in the text, of a designation, viz. the term 'Lord,' we understand that the highest Lord is meant 1.--But still the question remains how a certain extension can be attributed to the omnipresent highest Self.--The reply to this is given, in the next Sûtra.
196:1 This last sentence is directed against the possible objection that 'sabda,' which the Sûtra brings forward as an argument in favour of the highest Lord being meant, has the sense of 'sentence' (vâkya), and is therefore of less force than linga, i.e. indicatory or inferential mark which is represented in our passage by the angushthamâtratâ of the purusha, and favours the gîva interpretation. Sabda, the text remarks, here means sruti, i.e. direct enunciation, and sruti ranks, as a means of proof, higher than linga.