Size (large, small, medium) is extremely relative when dealing with astrological bodies, especially something as massive as a black hole. That said, up until a few days ago scientists had only found small and large black holes. Large black holes are thought to be at the center of every galaxy, and small black holes are thought to be formed from dieing stars.
Medium represents a new class of black holes. While not much is known about them, they may provide some evidence to support how larger ones are formed. By the merging of smaller ones that is.
In case your wondering, 'spotted' is different then 'seeing' when it comes to finding these behemoths. Trapping all light makes them as black as you can get. Seeing them is therefor impossible. Spotting involves detecting the effects of the massive gravity well which scientist have done.
"Only two sizes of black holes have ever been spotted: small and super-massive. Scientists have long speculated that an intermediate version must exist, but they’ve never been able to find one until now.
Astrophysicists identified what appears to be the first-ever medium-sized black hole, pictured in an artist’s rendition above, with a mass at least 500 times that of our Sun. Researchers from the Centre d’Etude Spatiale des Rayonnements in France detected the middling hole in a galaxy about 290 million light-years from Earth.
Called “Hyper-Luminous X-ray Source 1,” the structure sits on the edge of galaxy ESO 243-49. Because of the source’s physical characteristics and the pattern of its radiation, the researchers conclude it must be a black hole more than 500 times the mass of the sun: not too big, not too small, and the first of its kind."