What exactly did Lord Buddha say about Size of Atom? I have heard that Lord Buddha wrote or discussed about the smallest indivisible particle i.e the atom. What exactly did he say. Is it mentioned in some scriptures? If yes, did he give any argument or detail of his observations.
As described in Theravada Buddhist texts of the Abhidhamma Pitaka in the Tipitaka, there is no indivisible part of the atom. Every atom is divisible. If anyone divides it infinitely, the ultimate outcome is energy = there is nothing that exists in reality, it's a mirage that we experience because of the four major qualities. And the atom is made out of four major qualities and due to those 4 major qualities, there are another four qualities are generated. The four major qualities can exist in different or equal proportions within the atom. When the proportions changes, the elements change to another element. The four major qualities are:


"patavi" - which gives the quality of roughness to the atom
"aapo"- which gives the quality of flowing (fluidity)
"thejo" - the quality which gives warmth or heat (and this accounts for the coldness too, when the heat decreases we humans feel coldness, so it is both heat and cold)
"waayo" - the quality of expansion.


Then because of those four, another sub qualities being generated:


"warna" - the quality which gives colour to atoms
"ghanda" - which gives smell
"rasa" - which gives the quality of taste
"oja" - the energy that keeps those four main qualities intact (it's like food to the atom).


And the proportions of the four major qualities in atoms are constantly changing, the change can take fraction of second in some materials, sometimes it might take uncountable years to change. And according to Buddhist texts the atom itself appears and disappears very fast and we humans can only see its existence if you don't have developed your mind to understand and see those happenings (just like our human eye see the tv screen or a video as a one motion, it actually contains many frames to make a full video, but we only see it as a one frame with people moving inside it), but Buddha have said this is a mirage we see and it is "non-eternal". 

Source: I have read Abhidhamma Pitaka. And I'm a Buddhist.

P.S: if you want to know the what's really happening, my advise is, you have to read it by yourself and understand, rather than reading some English translated text.