The smallest dog I've seen mount a girl is actually... A Chihuahua!! It was the hardest thing ever; First she needed a box to get him to an 'okay' height. Her pose was the hardest Yoga shit; she had to spread her legs wide open, like really open, almost an split; her chest in the floor, and had to get her chest closer to her knees. Then she sticked her pussy to his dick, and all of that, just for him to be able to have the chance to fuck her, getting his dick inside is another story. At the end he fucked her and she had two orgasms, which was very shocking... She said she didn't enjoy the dick itself (Obviously), but that she imagined that the Chihuahua dominated her, took her against her will and bred her. She said that being his bitch turned her on, the concept that even the smallest dog could dominate her, fuck her and inseminate her was a turn on. So yes, even the smallest of the dogs can pleasure and fuck a girl in doggystyle. So definitely all dogs can mount, but not all girls. I think it's important to say that my friend is very fit and slim, she's154cm tall, and has this perception of herself, that let her reach those orgasms.
That being said, that's not optimal. The optimal would be that the dog can get his back paws around your butt with easy. So medium-large size dogs and further are perfect size.