It is said that Jupiter is a failed star. It is so massive that, if it were a couple of times more massive, it would have become a star. Stars can be way smaller than our Sun. For example, theoretically, a red dwarf star has only about 7.5% the mass of the Sun.
Poxima Centauri, another example, has only 12.3% the mass of the Sun.
The smallest possible star would be only about 50% larger than Jupiter. Imagine how our sky would be with a planet-star our there. Cooler than Tatooine from Star Wars, of course.
The smallest known star is MASS J05233822-1403022 at about 9% the mass of Sun.
Bonus: what is the biggest star in the Universe? The most massive we know of is R136A1 which is over 265 solar masses. Eta Carinae is also big: 120 solar masses. Betelgeuse, from Orion Constellation, a red supergiant star, with a radius of 980-1200 times the radius of the Sun.
Betelgeuse would be so big that even Jupiter’s orbit would fit inside it. Vy Canis Majoris, another great star, is 1300 – 1540 times the radius of the Sun.
See how big is Betelgeuse:
How big is Vy Canis Majoris compared to the Sun?
And here are all of the biggest stars in one row: