Actually, just about any woman can take 8x6. It's just that they have to be ready for it. To get a girl ready, just start gently bumping her labia. let her know that she doesn't have to take it to enjoy it, and that you are already enjoying it. All of a sudden she starts to dialate. It's not something that most women can do willingly. But once they've taken it, they really enjoy it. matter of fact, their orgasms are so strong with one that's bigger than they expected, that they want to go again, and again, and again.
They are the ones who have to accomodate you, and they are the ones who can get hurt, so it's normal that they would express concern, or dislike, or refusal. So the thing to do is not to worry about it. Nature will take its course as long as you are not anxious or worried. The trick is to put her at ease and just have fun. Don't get serious and start pounding her. She has to get ready, not you.
I would also add that it would be wise to practice multiple orgasms or restraining orgasm, so that you are not ever past the point. You must have control, all the time. If you are in charge, and she feels the calm restraint and desire to give her pleasure, then she will "open up" to you.