Welcome to the boards!
Before I say anything else, I just want to remind you that sex is much more than just intercourse.
If and when intercourse doesn't work between us and a partner, or isn't wanted at a given time by one or both people for any reason, that doesn't mean we can't have sex, it just means we can't have that
kind of sex. So, in the event that it does turn out that intercourse doesn't work for you and this partner, sometimes or even all the time, you still have a lot of other ways you can be sexual together: for more, see:
What's Sex? I hope by your partner is also aware of this by now!
That begs the question: have you two talked about this and the myriad of other ways you can be sexual together if and when intercourse doesn't work for you? What has he done in previous relationships: surely since this has come up before, he's had experience with other kinds of sex that have generally worked for him and his partner? Does he know that there are more ways to be sexual than just this? I know it might sound silly to ask that about people in our age group, but honestly, some people our age, especially cishet men, don't always know that intercourse or oral sex aren't the only ways to have sex. The good news is that when they learn, it's pretty much always a great thing for everyone, especially for them and their partners, since it will always expand what was otherwise a really limited sexual life. <3
The length of his penis shouldn't be an issue, since it's up to him or you how much of it is inside your body. Girth is something he obviously doesn't have that same kind of control over, but without knowing what we're actually talking about in that department, since "crazy" isn't an actual unit of measurement

it's hard for me to say how much of an issue that might be causing here.
Given your age, I do also want to check in about post-menopausal (or peri-menopausal, in the event you haven't reached menopause) vaginal care. Are you already regularly using either vaginal estrogen, a vaginal moisturizer (which is not the same as lube), or both? If not, less vaginal flexibility than earlier in life can also play a part with issues with intercourse like this.