If the knot swells inside a human hole that is too small then it would be uncomfortable, but how sensitive the dog is to that is an individual thing. It is not dangerous for them. The knot would just fail to grow to full size and the dog might get restless. This can happen when not taking the dog deep enough and the knot swells in the anus, so my advice (if you're a man) is to always try to lean back and be relaxed so he can get all the way in. You don't need to worry about your deep insides not being big enough because it is very stretchy. Too dry, that isn't an issue because he supplies enough lubrication during sex. Wetting the entrance can help with the initial penetration which can be a bit dry for the first second or two before he cums enough.
Yes. Dogs new to sex can be goofy, clumsy, and scared. They can freak out and pull too much, trip and forcibly pull out, step on their tether (what connects you to him), etc. If he is big enough that a pull out would be difficult for you, then it is good to do a couple of test ties where you give a handjob and try to hold the knot with your hand next to your hole for the full 15 minute tie. If you are big enough where pullouts are easy then it's no big deal if he messes up and no need to test. Someone said pullouts are uncomfortable for the dog and that is one way dogs learn to improve at staying tied. I haven't noticed that but it is a reasonable theory.