TOKYO, Jan 27 (Reuters) - The Mexican peso fell more than 0.5 percent against the dollar in Asian trade on Friday after the White House floated the idea of imposing a 20 percent tax on goods from Mexico to pay for a wall at the U.S.-Mexico border. peso MNX=D2 fell to 21.3300 to the dollar, stepping back from Thursday's three-week high of 20.8645 to the dollar.