My background is not in Advaita, I am from VishiShta Advaita. I read in one of the Upanishad commentaries by Sri Rangaramanuja Muni (who belongs to the Sri Vaishnava fold) that the Advaitins hold that due to avidya, both jivatvam (the quality of being a jiva) as well as aNutvam (atomicity) dawn upon the Brahman. Elsewhere, I read that the Advaitins hold jiva as vibhu as do other earlier schools of thought such as the Tarkikas, the Nayyayikas, and the Vaiseshikas. My question is, do the Advaitins attribute atomicity to jiva as long as Brahman sustains the jivahood? Or, do they not? At what points does this atomicity dawn and leave from Brahman? Are there any pramaNas from the Bhashya of Sri Shankaracharya?
The 'size' of the soul is discussed in Brahma Sutras Chapter 2, Section 3 Adikarana 13 (II.3.19-32) here with commentary (https://www.wisdomlib.org/hinduism/book/brahma-sutras):
Adhikarana summary: The size of the individual soul
19. (As the Sruti texts declare the soul’s) passing out, going (to other spheres) and returning (thence), (the soul is not infinite in size).
From this up to Sutra 82 the question of the size of the soul--whether it is atomic, medium-sized or infinite—is discussed. We have in the Svetasvatara Upanishad : “He is the one God . . . all-pervading” (6. 11); and again. “This Atman is atomic” (Mu. 3. 3. 9). The two texts contradict each other and we have to arrive at a decision on the point. Sutras 20-28 set forth the prima facie view. The opponent says, we find in the scriptures texts mentioning the soul’s passing out of the body, going to heaven etc., and returning from there. This is possible only if the soul is atomic, and not infinite or all-pervading; for to an infinite soul there can be no going and coming. Therefore the soul is atomic.
20. And the latter two (the going and coming) (being connected) directly with their agent (the soul), (it is of atomic size).
Even if the soul is infinite, still it can be spoken of as passing out of the body, if by that term is meant ceasing to be the ruler of the body. But the two latter activities, viz. the going and coming, are not possible for an entity that is all-pervading. So the soul is atomic in size.
21. If it be said (that the soul is) not atomic, as the scriptures state it to be otherwise (i.e. all-pervading), (we say) not so, for (the one) other than the individual soul (i.e. Supreme Brahman) is the subject-matter (in those texts).
Sruti texts like, “He is the one God . . . all-pervading” (Svet. 6. II), refer not to the individual soul, but to the Supreme Lord, who is other than the individual soul and forms the chief subject-matter of all the Vedanta texts; for that is the one thing that is to be known, and is therefore propounded by all the Vedanta texts.
22. And on account of direct statements (of the Sruti texts as to the atomic size) and infinitesimal measure (the soul is atomic).
“This Atman is atomic” (Mu. 3. 1. 9). Again we have, “That individual soul is to be known as part of the hundredth part of the tip of a hair divided a hundred times” (Svet. 5. 9), which shows that the soul is smaller than even the smallest. Hence the soul is atomic in size.
29. But that declaration (as to the atomic size of the soul) is on account of its having for its essence the qualities of that (viz. the Buddhi), even as the Intelligent Lord (Brahman, which is all-pervading, is declared to be atomic).
The word ‘but’ refutes all that has been said in Sutras 19-28, and decides that the soul is all-pervading, because the all-pervading Brahman Itself is said to have entered the universe as the individual soul, which again is stated to be identical with It. How then is the soul declared to be atomic ? Such declarations are on account of its preponderating in the qualities of the Buddhi (intellect) so long as it is imagined to be connected with the latter and in bondage. Passing out, going, and coming are qualities of the Buddhi and are only imputed to the individual soul. For the same reason also, i.e. limitation of the intellect, is the Atman regarded as atomic. It is like imagining the all-pervading Lord as limited for the sake of Upasana, devout meditation.
30. And there is no defect (in what has been said in the previous Sutra), (as the conjunction of the soul with the intellect exists) so long as the soul (in its relative aspect) exists: because it is so seen (in the scriptures).
An objection might be raised against what has been said in the previous Sutra that since the conjunction of the soul and the intellect, which are different entities, must necessarily come to an end some time, the soul, when so disjoined from the Buddhi, will either cease to exist altogether or at least cease to be a Samsarin (individualized). This Sutra replies : There can be no such defect in the argument of the previous Sutra, for this connection with the intellect lasts so long as the soul’s state of Samsara is not destroyed by the realization of supreme Knowledge. How is this known? It is known from the declaration of the scriptures that even at death this connection is not severed. “This infinite entity that is identified with the intellect .... Assuming the likeness of the intellect it moves between the two worlds, it thinks, as it were, it moves, as it were,” (Brih. 4. 3. 7). The terms “thinks, as it were”, “moves, as it were” also mean that the self does not think and move on its own account, but only through its association with the intellect.
Verse 23-28 have been omitted from the above. The verses quoted are the verses most relevant to your question. In verse 22, it says it is the smallest of the smallest, hence 'atomic' in size. The use of the word atomic should not be confused with the modern definition or size of physical atoms as defined by modern physics. The purpose of the verses is not to define a physical dimension to the atman, but rather to convey the idea that the atman is without physical dimensions - 'smallest than the smallest' - infinitesimally small.
In the subsequent verses it says how the infinite Brahman is defined as atomic within the individual soul. Many people confuse the word infinite as implying infinite physical space. Brahman is spiritual, not physical. The physical universe is in maya. Infinite space is maya. When thinking of the infinity of Brahman, think of Brahman as That which touches everything - hence infinite. Physical infinity has no meaning to Brahman. Physical infinitude is an aspect of maya. As the verses point out the infinite Brahman is atomic in size. When Brahman 'identifies' Itself with the individual Buddhi, it identifies Itself with the individual soul. Hence the Atman is both anu and vibhu. There is no conflict. The conflict is only an appearance of maya.
There are many attempts both physical(mostly in west) and intuitive(in east) to know the size of the soul. The western attempts are flawed. The question here is,"jiva aNu (atomic) or vibhu"?. As discussed in the Mundaka Upanishad Jiva=soul=Atma is atomic or very small: In the Mundaka Upanisad the measurement of the atomic spirit soul is further explained: eso 'nuratma cetasa veditavyo yasmin pranah pancadha samvivesa pranais cittam sarvam otam prajanam yasmin visuddhe vibhavaty esa atma. "The soul is atomic in size and can be perceived by perfect intelligence. This atomic soul is floating in the five kinds of air [prana, apana, vyana, samana and udana], is situated within the heart, and spreads its influence all over the body of the embodied living entities. When the soul is purified from the contamination of the five kinds of material air, its spiritual influence is exhibited." (Mund. 3.1.9) However, as I read the question whether it is Vibhu, I would think it is Vibhu also. My interpretation of word VIBHU is all pervading.I feel that it is the same principle of "living" which makes one to experience the universe, exist everywhere and hence it is Vibhu.
As per whatever I have learnt, athman is Anoraneeyan Mahathomaheeyan (kathopanishad). It is smaller than the smallest and bigger than the biggest. How is this possible?
Space time and causation form the objective world which is dreamt by you the athman. You create space in your dream and hence you are not bound by space. Calling yourself atomic or all pervasive are both incomplete and illusive. You are beyond space and time. Yet since it is a dream, the dreamer and the dreamt off is both the cosmic mind Hiranyagarbha and that is YOU! So all statements of athman being small or all pervasive are not entirely wrong but need some elaboration.