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As of 2008 the circumference of a regulation size baseball is between nine and nine and a quarter inches which converts to between about 7.3 cm and 7.5 cm in diameter according to the official rules of Major League Baseball (MLB).
That gave us today's 60 feet 6 inches. With an official diameter of 18 feet, and rising to 10 inches above the level field, the pitcher's mound, or hill, has been a part of baseball since around the turn of the 20th century.