HOUSTON/LONDON (Reuters) – BP Plc (BP.L) will face a maximum fine of $13.7 billion under the Clean Water Act for its Gulf of Mexico oil spill, several billion dollars less than feared, after a judge ruled that it was smaller than the U.S. government claimed. The ruling by federal magistrate Carl Barbier put the size of the worst offshore spill in U.S. history in 2010 at 3.19 million barrels. Under a “gross negligence” ruling Barbier issued in September, BP could be fined a statutory limit of up to $4,300 for each barrel spilled, though he has authority to assign lower penalties.