The observable level:
water --> water molecule --> atoms (two Hydrogen atoms and one Oxygen atom, size is about 1 Angstrom or 10^-10 meter)

Not directly observable level:
atom --> nucleus (10^-4 Angstrom) + electrons (10^-6M Angstrom) -->quarks (below 10^-8 Angstrom) --> Strings ? --> Planck length (10^-35 meter)

Up to the quark level, it has been verified. But even smaller scale, it is beyond the detection limit for now (therefore, I put a question mark over there). Hope we can get the answers in the future.

People used to think atoms were the smallest objects in the universe, but now obviously they are not. The smallest particles that have been found so far are quarks (protons, neutrons are made of quarks). A quark is no bigger than 10^-18 meter. Just to get an idea of how small this is:
Suppose we amplify a quark to the size of a grain of sand (10^-3 meter), and amplify a man by the same factor, then this man would be so big that he could swallow the entire solar system like an egg!

We don't know. There is a law in quantum mechanics that limits how small the location of something can be based on how quickly it may be moving, but this size is bigger than an atom for electrons, yet electrons fit into atoms.

There are theories about what quantum mechanics of space and time might look like, and they would be very small indeed, but it's hard to know whether these theories are right or not because we lack the technology yet do experiments to test them.

Things can get extremely small, and we now have many ways of observing these extremely small things! Things that make up our bodies, the surroundings and naturally occurring objects in space can get as small as 9.1 * 10^-31 kilograms (an electron). For reference, an apple has an average mass of 0.1 kilograms, so an apple, on average, has 10²⁹ times the mass of an electron.

Things that are classified as alive can get very small, too. A single bacterium has a diameter on the order of micrometers, which are one-one-millionth of a meter, or 0.00004 of an inch.