Bettina S., 17.03.21: I haven’t had any children. I had cancer surgery on the external genital area – the removal of tumorous tissue – but no chemotherapy or radiotherapy, and no narrowing due to scar tissue.
My previous partner (for 28 years) could only reach orgasm if I kept all my muscles tightly clenched. That meant I was never able to let go, never had an orgasm during intercourse, eventually stopped reacting due to the lack of foreplay, and had pain. For the last five years, I refused sex altogether.
Now I’ve met a kind, sensitive man – but he has a large penis. Unfortunately, intercourse is now almost impossible, or only possible with severe pain. After a long warm-up phase, 38 mm in diameter is the absolute limit I can manage with any tool. I find it hard to believe that my pelvic floor could be that strong, especially since I have trouble holding in urine even with a mild cold.

Answer: From what you’ve described, the cancer surgery itself doesn’t seem to be physically relevant to your current difficulties. What stands out much more – at least to me – is the emotional and sexual dynamic of your long-term relationship.
It seems the focus was mainly on your partner’s satisfaction. Over many years, constantly clenching during sex to help him climax – without attention to your own pleasure or comfort – likely led to a hypertonic pelvic floor. This means the muscles are not just tight, but chronically tense.
And such a pelvic floor is not strong – on the contrary, it can even contribute to incontinence.
I strongly advise against having vaginal intercourse in pain, even with your new partner. Doing so will immediately trigger a cycle: fear → pain → tension → more pain → more fear → more tension... and so on.
Instead, I wholeheartedly recommend regular use of FMS dilators. Start with a size you can comfortably insert. Gently move it back and forth to loosen your pelvic muscles. Over time, you can slowly increase the speed and/or the size – but only at your own pace. Focus on your breathing and give yourself time.
You should never go beyond your personal comfort zone. Mild stretching is fine – but pain is absolutely counterproductive.
The application method described here also applies to your situation:  https://theros.de/en/blogs/advisor/vaginismus-and-vaginal-cramps
Please also read the user instructions – they include helpful advice about your mindset during use.
Wishing you courage and success.