The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
25. But with reference to the heart (the highest Self is said to be of the size of a span), as men are entitled (to the study of the Veda).
The measure of a span is ascribed to the highest Lord, although omnipresent with reference to his abiding within the heart; just as to ether (space) the measure of a cubit is ascribed with reference to the joint of a bamboo. For, on the one hand, the measure of a span cannot be ascribed directly to the highest Self which exceeds all measure, and, on the other hand, it has been shown that none but the highest Lord can be meant here, on account of the term 'Lord,' and so on.--But--an objection may be raised--as the size of the heart varies in the different classes of living beings it cannot be maintained that the declaration
of the highest Self being of the size of a thumb can be explained with reference to the heart.--To this objection the second half of the Sûtra replies: On account of men (only) being entitled. For the sâstra, although propounded without distinction (i.e. although not itself specifying what class of beings is to proceed according to its precepts), does in reality entitle men 1 only (to act according to its precepts); for men only (of the three higher castes) are, firstly, capable (of complying with the precepts of the sâstra); are, secondly, desirous (of the results of actions enjoined by the sâstra); are, thirdly, not excluded by prohibitions; and are, fourthly, subject to the precepts about the upanayana ceremony and so on 2. This point has been explained in the section treating of the definition of adhikâra (Pûrva Mîm. S. VI, 1).--Now the human body has ordinarily a fixed size, and hence the heart also has a fixed size, viz. the size of a thumb. Hence, as men (only) are entitled to study and practise the sâstra, the highest Self may, with reference to its dwelling in the human heart, be spoken of as being of the size of a thumb.--In reply to the pûrvapakshin's reasoning that on account of the statement of size and on account of Smriti we can understand by him who is of the size of a thumb the transmigrating soul only, we remark that--analogously to such passages as 'That is the Self,' 'That art thou'--our passage
teaches that the transmigrating soul which is of the size of a thumb is (in reality) Brahman. For the Vedânta-passages have a twofold purport; some of them aim at setting forth the nature of the highest Self, some at teaching the unity of the individual soul with the highest Self. Our passage teaches the unity of the individual soul with the highest Self, not the size of anything. This point is made clear further on in the Upanishad, 'The person of the size of a thumb, the inner Self, is always settled in the heart of men. Let a man draw that Self forth from his body with steadiness, as one draws the pith from a reed. Let him know that Self as the Bright, as the Immortal' (II, 6, 17).
197:1 I.e. men belonging to the three upper castes.
197:2 The first reason excludes animals, gods, and rishis. Gods cannot themselves perform sacrifices, the essential feature of which is the parting, on the part of the sacrificer, with an offering meant for the gods. Rishis cannot perform sacrifices in the course of whose performance the ancestral rishis of the sacrificer are invoked.--The second reason excludes those men whose only desire is emancipation and who therefore do not care for the perishable fruits of sacrifices.--The third and fourth reasons exclude the Sûdras who are indirectly disqualified for sâstric works because the Veda in different places gives rules for the three higher castes only, and for whom the ceremony of the upanayana--indispensable for all who wish to study the Veda--is not prescribed.--Cp. Pûrva Mîmâmsâ Sûtras VI, 1.