Due to anatomical differences between woman and bitch, the chances of anything worse than "YIKES! THAT SMARTS!" (unless you/he have bad aim and he ends up in your ass instead of your vagina, or you're INCREDIBLY tiny "down there") is so vanishingly small as to not be worth bothering to speak of. If you had an "ouch" moment the first time you ever allowed a man to put his penis into you, being with a dog of anything below "ridiculously huge" size is HIGHLY unlikely to be any worse than whatever "ouch" you had then, and due to how a dog is built and how they operate sexually, it's more likely it will be less, if it's painful at all.
TL/DR: Pick pretty much any breed other than one of the truly huge giants, and you should be fine unless you're a "Tinkerbell"-sized gal.
What breed is recommended for a newbie to get knotted by? I'm thinking German Shepherd, but I'm open to suggestions. This would be a long term thing hopefully where I submit to him and become his bitch.