What is the size of the average thing in the Universe?
(WNYC Radiolab podcast; hyperlink at bottom.)
It is, of course, difficult to tackle this properly in only half an hour and for a pretty wide audience.
According to the way the question was posed, a thing is something unitary:
a star is a thing, a galaxy is not.
So, is a gas cloud a thing?
If not, when exactly does it become a thing as it condenses into a star?
(Note: both the Planck length and the estimated size of the Universe cannot really be considered sizes of things, but rather dimensions of the fabric of the Universe (metaphorically speaking).)
What is the size of very small things?
Quantum objects are nothing like solid bodies and defining size for them is non-trivial, if at all meaningful.
(Note: natural language does not have suitable words for describing quantum objects (for obvious reasons); the language of a branch of mathematics (functional analysis) does, but few people learn it.)
What is average?
Arithmetic mean, geometric mean, median?
The mode of a unimodal distribution?
Typical (an informal notion)?
Then, do we want a weighted average?
Do we accept a mean on a logarithmic scale (the mean of powers, i.e. exponents)?
The notion of a right answer depends on answering the above (and probably more) and that is quite some endeavor.
Now, ignoring all or most of that, in the podcast they approached the question twice and both times they arrived at the size of a big eucariot cell, about 0.1 mm (about 1/250").
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