Katha-2-3-17-aṅguṣṭhamātraḥ

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Now the sruti says this, for the purpose of concluding the drift of all the vallis. The Purusha of the size of a thumb, the internal âtman, as previously explained, is seated in the heart of all men. One should draw him out, i.e., separate him from one’s own body; ‘like what’ is explained. As the stalk within the grass, from the grass boldly with self-composure. Him, so drawn out from the body, one should know as Brahman previously described, pure, immortal and mere intelligence. The repetition and the use of the particle ‘iti’ are to show that the Upanishad ends here.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda