C-type asteroid
The largest unequivocally C-type asteroid is 10 Hygiea, although the SMASS classification places the largest asteroid, 1 Ceres, here as well, because that scheme lacks a G-type.
1299 Mertona
According to the surveys carried out by the Japanese Akari satellite and NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, "Mertona" measures between 14.14 and 14.60 kilometers in diameter, and its surface has an albedo between 0.219 and 0.243. Although such a high albedo is typical for stony asteroids, the "Collaborative Asteroid Lightcurve Link" assumes an albedo of 0.057, which it uses as the generic albedo for all carbonaceous C-type asteroids. It therefore calculates a larger diameter of 27.90 kilometers (as the lower the albedo or reflectivity, the larger a body's diameter at an unchanged absolute magnitude or brightness). Carbonaceous asteroids are the predominant type in the outer main-belt, while stony asteroids are mostly found in the inner regions of the asteroid belt.