The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
29. On account of the manifestation, so Âsmarathya opines.
The circumstance of the highest Lord who transcends all measure being spoken of as measured by a span has for its reason 'manifestation.' The highest Lord manifests
himself as measured by a span, i.e. he specially manifests himself for the benefit of his worshippers in some special places, such as the heart and the like, where he may be perceived. Hence, according to the opinion of the teacher Âsmarathya, the scriptural passage which speaks of him who is measured by a span may refer to the highest Lord.