Smallest thing in the world? Well Chinese people are always telling my how small my home town is, so I'm going to say "my home town".
The biggest thing? I think the Akita is a really big dog, so I'll say "an Akita".
The smallest thing in the world (i.e., on earth) has yet to be discovered.
The biggest thing in the world might be the Pacific Ocean.
This is an impossible question to answer, because it is impossible to know what should count as "a thing." And it is difficult to know what you mean by "the world." If you want a serious, literal answer, you need to construct a careful, technical question.
For example, "a university" might have over a hundred buildings built at different times over a period of more than a century. Is "a university" one big thing or one hundred smaller things?
To answer a different question smallest <em>living</em> thing is probably "a hepadnavirus." Definitions, again: it's small, but is it "living?" The biggest animal is probably "a blue whale." The biggest living thing might be Pando, a clonal colony of 47,000 quaking aspen trees in Utah; all of the roots of all the trees are joined so it might be considered to be a single living organism. Definitions, again: it is living, but is it one thing or 47,000 things?
It is tempting to look for a philosophical answer, a joking answer, or a poetic answer, because a simple factual answer seems impossible. I think I will end by quoting a poem by Emily Dickinson:
The brain is wider than the sky,
For, put them side by side,
The one the other will include
With ease, and you beside.
The brain is deeper than the sea,
For, hold them, blue to blue,
The one the other will absorb,
As sponges, buckets do.
The brain is just the weight of God,
For, lift them, pound for pound,
And they will differ, if they do,
As syllable from sound.
Yeah I think that's the common definition. But it's not an exact enough term to give a scientific answer to. For example, Wikipedia says "In a philosophical context it may refer to the whole of the physical Universe". Maybe Hard can clarify her intentions, or maybe I'm taking this too serious :-)
Just for clarity purposes, the 'world' is defined as the planet earth.
I suppose that includes the atmosphere up to the point were the vacuum exists.
just saying ... :)