Hello, my name is gary and i spent 10 years being used as an animal sex slave fucking my Mistress' dog. i've never had sex with a spayed female dog but i've provided a link you may be interested in reading. i personally advise against having sex with a spayed dog as does the site i've provided you with. Most medium to large sized dogs are able to handle a human's penis. A dog's penis thickens and normally doubles in length once he's knotted with a female. The knot is what "ties" or "locks" the male dog's penis inside the female while he's having an orgasm and the knot can reach the size of a fist or orange depending on the type of breed. Most dogs will stay knotted with the female for a good 5 to 15 minutes and he's cumming during this whole period. A dog's body temperature is higher than a human's as is it's cum, i've had Women who've had sex with dogs tell me this is the best part as they can feel this hot liquid shooting into them like a squirt gun.

But anyway, with a spayed dog, her sex organs have been removed and if the dog has never been into heat or had sex, you may never get her interested in this activity. Even with a dog that hasn't been fixed, it takes time and trust for the animal to allow you to have sex with her when she's not in heat. i'm talking years here, most dogs only go into heat 2 times a year. As a slave who was chastised and only allowed to fuck animals, this was extremely agonizing for me. But then, this was also a part of what my Mistress had in store for me when She did this to me. Well, here's the link. i hope it helps:

http://www.zoophile.net/howto-k9-sex-07.php