Not a female, but there are a few things about dogfucking in fiction that always annoy me. As jaeangel said, dog penises are shorter and thinner than would be that of a man (boy) of the same weight. The knot thing is grossly overrated -- I've experience of two male dogs with erections, and neither had a noticable knot. When a dog cums, it's one squirt. That's all. The thing is, in five minutes the dog wants to go again. One squirt. Then five minutes later...repeat ad nauseum.

My mother breeds collies, and her males are so inept that they can rarely get it in a bitch, even one that co-operates, even with two humans helping. If they can get the knot in, then they're stuck together for ten or fifteen minutes, and you'll likely get a good breeding; if they can't, they'll slip out every third stroke. For that reason, a realistic depiction of dogfucking should probably include frustration at how often the dog slips out. Note that training/experience would probably alleviate this problem.

Since dog vaginas are also proportionately tiny (and that's what the knot was evolved for), my suspicion is that the whole knot thing would probably only be effective on an under-aged virgin, but I can't speak authoritatively on that subject.