how small are atoms?

Why are atoms so small? To begin with, they are very small indeed. Every little piece of matter handled in everyday life contains an enormous number of them. Many examples have been devised to bring this fact home to an audience, none of them more impressive than the one used by Lord Kelvin: Suppose that you could mark the molecules in a glass of water; then pour the contents of the glass into the ocean and stir the latter thoroughly so as to distribute the marked molecules uniformly throughout the seven seas; if then you took a glass of water anywhere out of the ocean, you would find in it about a hundred of your marked molecules

--Erwin Schrodinger 'What is Life?'(1944)ISBN:0521427088

Of Many Worlds in this World

Just like as in a Nest of Boxes round,
Degrees of Sizes in each Box are found:
So, in this World, may many others be
Thinner and less, and less still by degree:
Although they are not subject to our sense,
A World may be no bigger than Two-pence.
NATURE is curious, and such Works may shape,
Which our dull senses easily escape:
For Creatures, small as Atoms, may be there,
If every one a Creature's Figure bear.
If Atoms Four, a World can make, then see
What several Worlds might in an Ear-ring be:
For, Millions of those Atoms may be in
The Head of one small, little, single Pin.
And if thus small, then Ladies may well wear
A World of Worlds, as Pendents in each Ear.

-Margaret Cavendish (1623-1673)