I've already covered what is the Largest star even though that was last year so its time to cover the smallest star. By the way, the current holder of the largest star is UY Scuti, a star in the constellation of Scutum.
There are so many contenders and if you Google the question, they won't all agree on the same star unlike the general consensus of UY Scuti. There are pages that will say that 2 Mass J0523-1403 in the constellation of Lepus is the smallest. Another contender to the smallest star is OGLE-TR-122 which has a planet in orbit round it in the constellation of Carina . 2Mass article is more recent so it could therefore be considered that the 2Mass is the smaller of the two stars because its more up to date.
Neither of those stars mentioned in the article above are visible with the naked eye on Earth because they are so small. The smallest star that is visible from Earth with the naked eye is 61 Cygni a double star in the constellation of Cygnus, the swan. Ref: Universe Today
The smallest group of stars are the red dwarf stars of which the nearest star to Earth other than the Sun falls into that group of stars, Proxima Centauri. Even though Proxima is the nearest star to us, it is not possible to see without a telescope. Its also been discovered that the star has a planet in orbit round it. Scientists find it easy to find exoplanets round red dwarf stars than they do with larger stars such as Hamal and Aldebaran in the constellations of Aries and Taurus respectively.
The reason why planets are easier to find around smaller stars is because planets have more of a visible effect on small stars compared to larger stars. The visible effect could be to cause the star to wobble, i.e. move or blink, i.e. transition. It is even believed that the planet around Proxima Centauri may even be hospitable for life to exist.
A star that is marginally (16%) bigger than Jupiter has been discovered and that star is OGLE-TR-122. The OGLE is smaller than some exoplanets which can be thirty percent larger than Jupiter. According to Wiki
According to Phil Platt, the professional astronomer behind Bad Astronomy is on record as saying 2Mass can fit inside Jupiter which would therefore make it a didler compared to other stars. It is believed it might be the smallest a star can get so it could not only rank as the smallest star so far discovered but also the smallest star in the universe. Jupiter has sometimes been called a failed star so why if the smallest star is smaller than Jupiter, is Jupiter not a star. Its all about mass, a gas giant has to have sufficient mass in order to start nuclear reaction and Jupiter is just not massive enough. Ref: Curiosity
The only small stars that have been discovered so far are close-by stars, the 2Mass star is a mere 40 light years from Earth. Anything smaller if there is a smaller star would be marginally smaller if anything and absolutely nothing in it.