* U.S. estimated 4.9 million barrels of oil spilled * BP sought evidence that may show spill was smaller * Clean Water Act fines dependent on spill total By Jonathan Stempel April 11 (Reuters) - BP Plc will gain access to U.S. government documents that may shed light on the size of the 2010 Deepwater Horizon oil spill in the Gulf of Mexico, a crucial issue in determining the oil company's liability.