Smaller than an atom, larger than the earth

अणोरणीयान्महतो महीयानयमव्ययः |
गुहायां निहितश्चापि जंतोरस्य महेश्वरः || 24 ||

 

Smaller than an atom, larger than the earth, undecaying, ensconced in the heart of living beings, even though the great God of this (everything).

 

Part 1, Chapter 2, shloka 20 of Katha Upanishad is very similar.

 

अणोरणीयान्महतो महीयान्

आत्मस्य जन्तोर्निहितो गुहायाम् |