Probably "strings" but until that is proven quarks or leptons.
"a"
Smallest and nearest are two different criteria. See related questions.
The solar system is the smallest
A star - especially as a galaxy is formed of billions of stars.
The smallest observed thing in the galaxy are Quarks. However, there are postulated other particles that seem likely to exist but are yet to be observed. given the resent success of the large hadron collider in the coming years it is entirely possible more will be discovered.
Mars is not a galaxy! It is a planet in our solar system.
No. In fact, it is one of the largest we know of.
no.
In order from smallest to largest: Solar System, Galaxy, and Space.
Higgs Boson -> Galaxy Filaments.
Universe, galaxy,nebula,solar system, star, planet
The cell is the smallest living thing and an atom is the smallest non-living thing.