A regulation baseball is 9 inches around and weighs approximately 5 ounces. Many leagues use safety or "RIF" (reduced injury factor) baseballs, at least in T-ball and the lower "minor" leagues. Because safety baseballs are softer than regular youth baseballs, they don't hurt as much when they hit a player. A study [1] by the Institute For Preventive Sports Medicine [2], headed by MomsTeam.com expert, Dr. David Janda [3], has shown that softer baseballs reduce the number and severity of head injuries, but another study [4] by Dr. Janda shows that, contrary to the claims of some manufacturers, soft baseballs do not reduce the risk that a player struck in the chest by a ball will suffer commotio cordis [5], a potentially fatal cardiac injury. The use of soft baseballs will help players from developing a fear of the ball, which, once instilled, is difficult for a player to overcome.
Links:
[1] http://www.ipsm.org/publications/softball_injuries/head.asp
[2] http://www.ipsm.org/
[3] https://mail.momsteam.com/node/134
[4] http://www.ipsm.org/publications/softball_injuries/blunt.asp
[5] https://mail.momsteam.com/node/301