The nature of this Atma!

 Look at this beautiful statement from the Chandogya Upanishad: 

एष म आत्मान्तर्हृदयेऽणीयान्व्रीहेर्वा यवाद्वा सर्षपाद्वा श्यामाकाद्वा श्यामाकतण्डुलाद्वैष म आत्मान्तर्हृदये ज्यायान्पृथिव्या ज्यायानन्तरिक्षाज्ज्यायान्दिवो ज्यायानेभ्यो लोकेभ्यः ॥ ३.१४.३ ॥

eṣa ma ātmāntarhṛdaye'ṇīyānvrīhervā yavādvā sarṣapādvā śyāmākādvā śyāmākataṇḍulādvaiṣa ma ātmāntarhṛdaye jyāyānpṛthivyā jyāyānantarikṣājjyāyāndivo jyāyānebhyo lokebhyaḥ || 3.14.3 ||

• एष म आत्मान्तर्हृदये - this "aatma" at the core, in my heart
• अणीयान्व्रीहे वा यवात् वा सर्षपात् वा श्यामाकात् वा श्यामाक-तण्डुलात् — it is smaller than a grain of rice or a mustard seed or a millet or the husk of a millet. 
• एष म आत्मान्तर्हृदये - this consciousness within me, in my heart 
• ज्यायान् पृथिव्याḥ— greater than the earth
• ज्यायान् अन्तरिक्षात् — greater than the sky/atmosphere
• ज्यायान् दिवः — greater than the heavens
• ज्यायान् एभ्यः लोकेभ्यः — greater than all these worlds that we experience 

The self / spirit / consciousness within my heart is smaller than a grain of rice, smaller than a grain of barley, smaller than a mustard seed, smaller than a grain of millet, smaller than even the husk of a grain of millet. The same self / consciousness in my heart is larger than the earth, larger than the skies, larger than the heavens, and larger than even all these worlds combined.

A similar quote is found in Kathopanishad also: 

अणोरणीयान्महतो महीयानात्मास्य जन्तोर्निहितो गुहायाम्‌। तमक्रतुः पश्यति वीतशोको धातुप्रसादान्महिमानमात्मनः ॥

Smaller than the smallest, larger than the largest than, the self / consciousness. It hides in the secret heart of the creature! When one who is free from desires, devoid of sorrow, with the grace of the root (the Lord who is the primary cause of everything), sees the greatness of the self.

Just in awe of the thought - to perceive the same aspect in two extremes. May that "dhatu" bless us all. Hare Krishna!