From personal experience it’s usually small enough to push in, as long as my boy really gives a good thrust, otherwise it stays out. I feel its too risky to try, while not causing pain to either you or your pup, when its fully engorged I mean.
You generally you to get it in when it is small. Pushing in a dog's knot after it is grown can be uncomfortable for them depending on how tight a fit their dog's knot is and how much force you need to apply to get it in there.
it doesn't go in already big. canine penises don't work the way ours do. they don't go erect before getting in place. it's why they have a bone inside.