There is no drain plug.
More than likely 17 mm.
17MM
could be 15, 16, or 17mm wrenches
19mm
It would be a 17mm
17mm 17mm
14 mm
17mm
I believe the size is 14mm (thread size) and you use a 17mm box end wrench or suitable socket to loosen..unless some gorilla with a impact wrench tightens it and use can't get it off..like my 2002 Honda civic EX.
need to know what size tool to drain rear end on a Honda fourtrax
The size wrench to remove it should be 15mm.