This is all I could find after a quick google search. Nothing on the size of the baseballs they use.
There is no single, official ball sanctioned by either league or by the Commisioners office. Rather, the commissioners office certifies that the balls of different sporting goods manufacturers are eligible to be used in official games, provided they meet the commissioner's specifications. Each team is allowed to use balls, from up to three different makers, during each season for their home games. The home team must select the balls from one maker only to be used during each series and must supply the visiting team with samples of the balls to be used in advance.
The use of different maker's balls causes lots of difficulty in comparing the stats of different teams, since no manufacturer's balls are identical. In 1980, things got very wierd when one manufacturer's balls, which were particularly lively, were adopted widely by four Pacific League teams: the Hawks, Buffaloes, Braves and the Lions. This particular lively ball had been used for a few years, but when it went into wide spread use in 1980, home run totals jumped by 28% and while the total number of hits remained stable, doubles and triples were way down but homers were way up. Perhaps a lot of hard hit balls that would have hit the fences, were gliding over them. The home run explosion which swept the league, did not occur in the home parks of either the Lotte Orions or the Nippon Ham Fighters--who had opted not to use the juiced ball.
At the conclusion of that season, the commissioner's office instituted a much stricter testing regime with "random" testing taking place twice a year at by an independent testing agency in Saitama Prefecture. The balls are then tested for coefficiency of restitution, the measure of how much energy the ball retains from a collision. It is uncertain how random this testing is as the makers are asked to supply balls for testing, rather than having them taken from those already supplied to teams for game use.
Other than twice monthly tests in Osaka and Tokyo for weight and whiteness, which are conducted by umpires and performed prior to stamping each ball with the commissioner's seal, all other testing is done by the makers themselves. This system appears open for abuse.The fact that the Nippon Ham Fighters, playing in a decent home run park--but not a band box--see 70% more home runs in their home games suggests that the balls are a factor
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