List of largest stars

Below is a list of the largest stars, so far discovered, ordered by radius. The unit of measurement used is the radius of the Sun (696,392 km; 432,717.927 mi).
Additional methods.
A few stars are in the Zodiac, and the Moon sometimes passes in front of them. This allows calculating their size by their angular size and distance. This is not very reliable. Most do not, so astronomers calculate their size by their spectral type (which gives their luminosity), distance, and brightness. This is even less reliable.
Caveats.
These objects are extremely big, thousand to millions of times the volume of our Sun, and extremely luminous. These stars also have extended atmospheres and photospheres, and are often shrouded in dust (Like Stephenson 2-18, WOH G64 , UY Scuti and VY Canis Majoris). This makes their true size uncertain. Many of these stars vary in size and brightness (like Betelguese, Antares, UY Scuti and VY Canis Majoris).
These objects are also far away, sometimes intergalactic (eg: WOH G64 is in the Large Magellanic Cloud). This makes it even harder to calculate their sizes. Most stars found will not be above 1,500 times the Sun's radius. Their mass would hardly hold together, and a lot of material would be ejected by powerful solar winds, forming the nebulae we see around them.