I do not exactly know what you mean so:
1. If the knot inflates too much in the sheath it is ok. The sheath can take it, but taking the inflated knot out of the sheath is painful. So if you mess up and do not manage to get the penis out in time, wait for the erection to fade and try again later.
2. Knotting with humans does not happen very often. Knotting requires a special ring muscle in the vagina that holds the knot. Humans are not equipped with this accessory.
The only thing similar to this muscle a female dog has is the human anal sphincter which is still usually too weak to hold a knot.
So very often you will see a fully inflated knot slip out of a human vagina with basically no effort at all. Knotting in humans pretty much only happens when there is a larger size difference between the knot and the vagina. Large dog - tight human equipment.
Because of that, you might very likely need to use your hands to hold the knot in you. Do not hold the legs, or any other part of the dog, hold the knot. Pressure on the knot is key in keeping the dog erect and bringing pleasure. Lack of pressure on the back side of the knot signals end of sex and usually means the dog will jump off.
You often see this mistake in porn where you have a dog trying to mount a woman 15 times with no success because nobody is holding the knot and providing stimulation there which is a signal for a dog that he hit the right spot and things are going smoothly.