The drain plug is M14, ie you need a 14mm ID crush washer. A 9/16 washer will fit.
Officially, according to Triumph, a 14.7mm ID X 21.75mm OD X 2mm thick crush washer is the right one. Or whatever is really close.
EDIT: Looks we had a duplicate thread going! So I merged the two together. Lo and behold, identical answers, more or less sort of.