Once their cock tip feels your hole they do force it in, but..... it goes in small, basically just a hair bigger than their flaccid size upon entry. On rare occasions if they swelled up some a little soon, they might be half size when the knot first goes in.
The full size knot is NOT usually slamming the entrance to the vagina unless he accidentally pulls out on some of the thrusting. If the dog's penis is med to large, and your entrance is snug, rarely will a knot pull out in the 25 seconds of rapid thrusting. We do see it happen sometimes when the cock is smallish to medium size.
So, basically it will enter you being about 4" or 5", but inflate to large size within 3 to 5 seconds. It's truly amazing how quick they swell up.
A really wide knot can be accommodated more often than long penis length can, but sometimes it's astonishing to see that woman fit all the length too. At least the head of their penis is not wide, and it can tuck up there beside the cervix.