The Leroy Heiman chronology says 24' x 16':
"Leroy Neiman painted 24' x 16' portraits of Sylvester Stallone for Rocky films.
Neiman also appears as ring announcer in Rocky III, IV, and V".
http://www.leroyneiman.com/leroy-neiman-chronology.asp
Ruth Boaz says:
"Neiman also created an energetic piece that was used for the closing credits of Rocky III which captures the final frame of the film - when Rocky and Apollo Creed are just about to swap blows. This particular piece, an original, has never been reproduced or offered as a print, however the "Rocky Portrait" from other Rocky films are available as lithographs, and painted copies."
http://www.google.com/search?client=safari&rls=en&q=Leroy+Neimans+original+painting+dimentions+Rocky+VS+Apollo&ie=UTF-8&oe=UTF-8
Ruth Boaz speculates that the reason for this piece not being available in a lithograph, or even a serigraph, perhaps has to do with the possibility of the maker of the movie having bought the rights to the painting.
https://answersrip.com/question/index?qid=20071228132145AAgbtmi