There is no one distinct species of big cat called a "black panther." Panther is just an old general term that comes from the scientific name "Panthera," a genus that includes the jaguar, leopard, lion, and tiger. Over the years "black panther" has become a common name for any large black-coated cat. If you see a black panther, you are really looking at either a leopard or a jaguar with melanistic coloration. The term "melanism" is derived from the word melanin, a dark colored skin and hair pigment. In jaguars and leopards, melanism results in the fur being very dark or black. Often, the spots and rosettes of these animals can be faintly seen through their dark fur. Melanism occurs because of a dominant gene in jaguars, but a recessive gene in leopards. Although it is hereditary, melanism is not necessarily passed directly from one generation to the next. It's common, therefore, to see "mixed" litters with one black cub and one or more normally colored cubs.
As to size, black panthers are the same as the leopard or jaguar. The jaguar is generally considered the third largest cat in the world (after the lion and tiger). The largest jaguars grow to a length of about 6 ft. (1.8 m) with an additional tail length of about 30 in. (76 cm). The largest leopard's body length is nearly 5 ft. (1.5 m) with an additional tail length of about 3 ft. (91 cm).