Asteroid
Although their location in the asteroid belt excludes them from planet status, the three largest objects, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of "potato"-shaped asteroids. The fourth largest asteroid, Hygiea, has an undifferentiated interior, like the majority of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt.
Near-Earth object
Two of the biggest Near-Earth objects 433 Eros and 1036 Ganymed, were also some the earliest to be detected. 1036 Ganymed is about 20 miles in diameter. As bigger asteroids they reflected more light. One way around simply finding the NEOs that reflect the most visible light, is to use infrared telescopes that observe the objects heat emissions.