What's so big about "small objects?"
If an asteroids orbit brings it to within 0.05 astronomical units (AU) of Earth's orbit, it is categorized as "potentially hazardous" unless it has an absolute magnitude H greater than 22.0, which corresponds to a diameter on the order of 135 meters/yards (larger H means less bright, thus smaller size). And 0.05 AU is about 19.5 times the distance between Earth and Moon (0.00256 AU). To be discovered and tracked, such small objects usually must come close, and more than a hundred are already known to be among Earth's nearest neighbors, coming closer than the Moon.
These small to tiny objects are mostly exposed pieces of distant asteroid populations, although some may be remnants of Earth-Moon system collisions. They someday will be space mission targets and in-space resources. Some will end in spectacular atmospheric events after coming too close to Earth, possibly with some pieces surviving as meteorites. But today they zoom quietly and dimly past Earth in large numbers, and their occasional discovery and then follow-up represents the most difficult and very best asteroid observing work being done today by amateurs and professionals around the world.
Diameter & Earth MOID: Stated diameters are rough best estimates from a standard but very inexact H-to-size formula using H (absolute magnitude, or brightness) from the JPL NEO Orbital Elements page, source also for Earth MOID (minimum orbital intersection distance). Current Minor Planet Center H is also given, along with the original H from each object's discovery MPEC.
Sources: Planetary MOIDs are from JPL and Lowell Observatory, and flyby distances and times are from the JPL Close Approach Table. See also the Sormano Observatory SAEL page (for H>22.0 and Earth MOID<0.015 AU), and NEODyS listings have yet another H calculation.