My GF was happy with my average size cock when we first got together. She is Asian and never had children, so her pussy was very tight. We started swinging about six months after we got together and she would complain about any of the guys whose cocks were larger than mine, that they hurt her. So, for a while we deliberately looked for couples and singles where the guys had average or smaller cocks. After several years, we were traveling on the West Coast and passing near a town where a guy lived that we had been in contact with. He had a large, 9X6 inch, cock and his profile photo showed that he was very fit and very good looking. She liked his looks and the way he chatted in the emails, so we made arrangements to meet him.
By that time I had learned that if she had a good orgasm, her pussy seemed to get a little looser and very wet with her lube juice. The guy turned out to be very smooth and we both enjoyed him. He and I worked together to give her several orgasms, and then he slowly worked his large cock into her pussy. He had to stop at 6 inches deep, and she loved the way he filled. Later in the evening, she was able to take him anally, and she loved all of his cock deep in her.
It was a great MFM and since then she has loved to have big cocks. I do not worry. Her pussy and ass are still as tight as they were before the large cocks, and she is delighted that she can have then any time she wants. As a result, she makes sure that I know how happy she is by taking very good care of me.
Our recommendation is that if your relationship will accept it, try swinging and let her have cocks of whatever size she wants.