
The smallest known star right now is OGLE-TR-122b, a red dwarf star that's part of a binary stellar system. This red dwarf the smallest star to ever have its radius accurately measured; 0.12 solar radii. This works out to be 167,000 km. That's only 20% larger than Jupiter.
OGLE-TR-122b must be denser then Jupiter.
My reference was to the movie 2010, the sequel to the movie 2001, a Space Odyssey. At the end, Jupiter is turned into a small sun to warm Europa, which the aliens responsible for the black monolith have seeded with new life.