The technical capabilities of the Keyholes are classified; however, their resolution can be estimated from public information about the diameter of their primary mirror, and their orbital altitude.
Jeff Richelson, citing an interview with a former CIA official, reported, "the mirror for the first KH-11 was quite good and quite large - seven feet, eight inches wide. (Subsequently, mirror size increased)." (The Wizards of Langley, pg. 200)
Assuming that a mirror is diffraction-limited, then its resolution is a function of its diameter, the wavelength of light to be detected, and the distance from the target:
resolution = 1.22 * wavelength * distance / diameter
The wavelength of visible light is about 5.5e-5 cm. According to Richelson, the first KH-11's mirror was 234 cm in diameter. At perigee, a KH-11 is about 300 km above the Earth, or 3e7 cm.
The above formula yields 8.6 cm as a KH-11's resolution at 300 km, which rounds to 10 cm.