Free Willy:
Typically, if people are talking about girth without being more specific, it’s midshaft girth. Measuring base girth is worthwhile as well, though; it’s just more information, and that’s not bad.
I don’t think you would be doing badly if you keep the stretch maintained for the duration of the stretching exercise, but personally, I think I stretch, release stretch, change direction, stretch in new direction, release stress, etc.
I think doing the stretches in the hot shower is just fine. Doing them afterward is just fine too.
Just don’t lose your grip, resulting in your hand swinging up in an arc, striking yourself in the forehead, and knocking yourself out. That would be dangerous, especially in a hot shower. ;)