The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
13. And it is minute.
And the chief vital air is to be considered as minute like the other prânas.--Here also we have to understand by minuteness that the chief vital air is subtle and of limited size, not that is of atomic size; for by means of its five functions it pervades the entire body. It must be viewed as subtle because when passing out of the body it is not perceived by a bystander, and as limited because scripture speaks of its passing out, going and coming.--But, it may be said, scripture speaks also of its all-pervadingness; so,
e.g. 'He is equal to a grub, equal to a gnat, equal to an elephant, equal to these three worlds, equal to this Universe' (Bri. Up. I, 3, 22).--To this we reply that the all-pervadingness of which this text speaks belongs to the Self of the prâna in its adhidaivata relation, according to which it appears as Hiranyagarbha in his double--universal and individual--form, not in its adhyâtma relation. Moreover the statements of equality 'equal to a grub,' &c., just declare the limited size of the prâna which abides within every living being.--Thus there remains no difficulty.