Neat question! The largest star in the universe that scientists have identified so far is called R136a1 (Crowther et al., 2010). This star is in a nearby galaxy just outside of the Milky Way. It is ~320 stellar masses (Crowther et al., 2010); that’s 320 times bigger than the sun! The paper that I found this information in is very interesting. The scientists who estimated the size of the R136a1 also determined that its age must be about 1.5 million years (Crowther et al., 2010). That’s really young compared to other things in the universe! Our sun is about 4.5 billion years old; that’s 3,000 times older than R136a1! It turns out that stars are the biggest early in their “life cycle”, and they lose mass throughout their lives. It is certainly possible that there are larger stars in the universe; we just haven’t identified them yet!

reference
Crowther, P.A. et al. (2010). The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 stellar mass limit. Monthly Notices of the Royal Astronomical Society, 408, 731–751.

One of the biggest stars that scientists know of is called VY Canis Majoris. It is about 1400 times as large as our sun!

The largest star known is currently NML Cygni, which has a radius that would not quite reach the orbit of Saturn in our solar system.