return tile size as (width,height) in meters

def size(self):
		'''return tile size as (width,height) in meters'''
		(lat1, lon1) = self.coord((0,0))
		(lat2, lon2) = self.coord((TILES_WIDTH,0))
		width = mp_util.gps_distance(lat1, lon1, lat2, lon2)
		(lat2, lon2) = self.coord((0,TILES_HEIGHT))
		height = mp_util.gps_distance(lat1, lon1, lat2, lon2)
		return (width,height)