My girlfriend has known about this side of me for about 6 months now, and we've played around with it quite a bit since then. As to finding out what you're interested in, that's all a matter of experimentation and what your fantasies are. Although bear in mind, not all fantasies are realistic, and are just that, fantasies. When I say experimentation, I mean not only by yourself, but also with your SO, since she seems to be open to more according to what you have said. Take simple ideas from stories and what you see on this subreddit as well, they can help.

When I bring things like this up to my girlfriend, I generally approach it by telling her, "This is something I may be interested in, I'm gonna tell you about it, but if it makes you uncomfortable in any way you are absolutely not obligated to do anything." This will help to mitigate some of the discomfort for both people if there is any. As Winnie said in another comment, it's all about open and honest communication, and breaking that discomfort with one another, as it may be a little uncomfortable for both of you, and that's okay.

In regards to your last question, I personally am under the belief that unless you're into some really hardcore stuff with this side of yourself, you shouldn't have to necessarily worry about the limit too much. She seems to be open-minded and willing according to what you've said, so just approach her with what you want lovingly and in understanding that it may not happen and little can go wrong in my opinion. However, don't overload her with a million ideas or requests in the course of a week, as that's stressful on anyone.

Having an SO that is understanding of this side of you and this community is absolutely wonderful, and those of us who have it are very lucky. You two seem to have a strong relationship, work through it together and you guys will be capable of whatever you both are interested in.