Why rankings are hard
The edge of a red supergiant is not a sharp surface like a planet's ground. These stars have extended, moving atmospheres, dust, and changing brightness.
A small change in distance or temperature estimate can produce a large change in calculated radius.
Famous large examples
UY Scuti, VY Canis Majoris, Stephenson 2-18, Mu Cephei, Betelgeuse, and Antares often appear in large-star discussions.
The responsible answer is that several red supergiants and hypergiant candidates are comparable within uncertainties.
What a huge radius means
If a red supergiant with a radius of about 1,000 solar radii replaced the Sun, its outer layers could extend beyond the orbit of Mars and toward Jupiter.
That comparison is a scale model, not a stable solar-system scenario.
Brightest is not the same as largest
The brightest-looking supergiant in Earth's sky is not necessarily the one that releases the most energy. Apparent brightness changes with distance and dust, while intrinsic luminosity depends strongly on radius and temperature.
Betelgeuse and Antares are prominent red supergiants in our sky, but a more distant star can be intrinsically more luminous. Rankings should always name the measurement being compared.