The Nature of Atma is infinite. This infinite nature of Atman is stated In Svetasvatara Upanishad Chapter 3 -Verse 16 Page No.73 .Its said that Atma is pervading everything in the universe.
सर्वत: पाणिपादं तत्सर्वतोsक्षिशिरोमुखम् |
सर्वमावृत्य तिष्ठति ||16||
With hands and feet everywhere , with eyes , mouth everywhere ,with
ears everywhere .That exists pervading everything in the universe.
But as you asked specific question regarding its location , the answers are provided in Upanishads.
Here in Svetasvatara Upanishad Chapter 3 -Verse 13 -Page 70 - the exact location of Atma is given – It resides in the space of the Heart.
सदा जनानां ह्यदये संनिविष्ट:|
ह्युदा मन्विशो मन्सभिक्लुप्तो|
य एतद्विदुरमृतास्ते भवन्ति
Assuming the size of the thumb by virtue of intellect , emotion
imagination ad will the infinite being dwells in the hearts of the
creatures as their inner self. Those who realize it become immortal.
Also see the Answer given by The Prashna Upanishad
हृदि ह्येष आत्मा ।
अत्रैतदेकशतं नाडीनं तासां शतं शतमेकैकस्या द्वासप्ततिर्द्वासप्ततिःप्रतिशाखानाडीसहस्राणि
व्यानश्चरति ॥ ६ ॥
hṛdi hyeṣa ātmā | atraitadekaśataṃ nāḍīnaṃ tāsāṃ śataṃ śatamekaikasyā dvāsaptatirdvāsaptatiḥpratiśākhānāḍīsahasrāṇi
bhavantyāsu vyānaścarati || 6 ||
6 This âtman is in the heart. Here, there are a hundred and one
nerves. Every one of these has a hundred brandies; again, every one of
these has seventy-two thousand sub-branches. In these, vyâna moves.