A:AnswerI believe the 38-40 is for smaller wrists like mine. The larger 42+ size was large on me. The end of the strap came right to the bezel of the watch. So there was plenty of room for a larger wrist. I went back and got the 38-40mm one and it fits much better.
A:AnswerIt's a sturdy nylon cloth. I would say yes to washable.
For garden work, maybe consider the bands like the one that came with the watch. Very interchangeable and you won't have too wait for the nylon one to dry.