The problem is, dog penis can vary in size even among the same size dog and same breed dog.
For example:
You can have a 25k husky with a 20cm long penis and grape fruit size knot and 16cm circumference shaft. (aka massive penis)
Or you could have a 25k husky with a 14cm long, pool ball size knot and 12 cm circumference shaft. (aka small penis)
That female husky could also have variations on how much room she has in her vagina, she may have more or less room, she may also be more or less thigh.
If you want to know if a specific bitch would be vaginally compatible with your penis, well, first measure your penis circumference.
Then measure your bitch vaginal canal circumference. How do I do this? If she is ok with it, use lubricated fingers. Can she take 1 finger? 2 fingers? 3 finger? 4 fingers?
Then translate how many fingers she can take to circumference. Like, if she took 3 fingers right below the knuckles, then wrap a sting around your 3 fingers just below the knuckles and then measure the string with a ruler to see how much it is.
If she is say (example) a 12cm in vaginal opening (after measuring with fingers) and your circumference is 12 or less, then you are half way there. Now is just a matter of her consenting to penetration and consenting to humping. FYI: A bitch may consent to be penetrated without problems, but may still deny consent when you start humping her. Obviously, you have to stop is she starts whining during the humping part of sex.
What if you are say 14cm circumference and she is 12 cm in opening? Well, from personal experience, if you are under 2cm difference. You can still try to go inside her while being soft, and then get erect inside her. Just like a male dog would do, male dogs go inside a bitch non-erect and get big inside the bitch.
Again, read all the signs that says "NO I don't consent" and be careful, use plenty Ky-lubricant. Have clean hands, nails short and sanded, clean dick (wash it before sex) make sure that she enjoys it or that at bare minimum she is neutral to it. If she finds sex to be a neutral experience, make sure to make it more fun with compliments and tasty treats during or after sex.
If your bitch is too tight, which is almost always the case for girl dogs that are virgins (as non-virgin dogs gets their insides expanded forcefully and painfully during mating and later during birth) you could try doing stretching exercises.
These exercises are done by progressively inserting more and more lubricated fingers inside her every day for 5 minutes (can be done once a week too, but this would take you 4 times longer) For example: Say she only can only take a pinky finger, well, start by gently inserting a thicker finger. Once she can handle that, try inserting one finger and then try pushing for a second, once you feel she expanded a bit, hold it there and wait 5 minutes and then remove them, try the same again tomorrow. Repeat this and slowly but surely, she will be taking more and more fingers, till she can take 2 or 3 or 4 fingers in her.
Also, in my humble opinion, don't over expand to extremes. It could lead to risk of prolapse. I remember some one once told me that he spend a WHOLE YEAR expanding a dachshund to have sex with her because he loved her, expanding such a small dog to accommodate and average human penis does not sound safe at all, nor sound like something you would do out of love.
