Experience dictates that foreplay is the answer, and foreplay can take any number of forms. The ultimate object is to get your partner to the point where she wants you in her body - where her desire is so intense, she can literally taste it; wetness, openness and heat follow naturally. Granted, this takes time, patience and practice, but the rewards are always well worth it. If you can get your partner into that state of mind, and it is more a state of mind than a state of anything else, it matters not what your size is, the most extreme anomalies possibly excepted.
When I was younger (I am older than most trees), I found that some women - invaginated my penis as much out of a sense of duty as anything else. Some felt it was a challenge. I can think of several women who refused when they saw it, or tried and couldn't....I still did my best to make it a memorable experience and when they realized that I was doing this as much for their pleasure as my own, many of those who couldn't suddenly could. I was with her and I felt I might as well make the most of this experience and if generally worked out well. Sadly, as you will see below, I was not always this way.
Strangely, though, my ex-wife was one of the few who could, but who never really adjusted to my size, despite us being together for five and a half years. On the other hand, I fondly remember a very tiny 20 year old Asian woman (all of 4'10" and maybe 85 pounds soaking wet) who took everything I had - with great joy - on the very first try. I am convinced it is all in the mind....hers, yours and collectively. Age, body size, body type, race, whether she has given birth and even her experience have little to do with this. Now, if she is genuinely in pain, i.e., you start banging into an ovary - or both - it is time to withdraw. She will appreciate this. On the other hand, if you play Diesel dork and go on to your own orgasm... well....it's almost certain that she will not desire you as a sex partner again. I speak with experience on this point, as I did this on one memorable occasion when I was young; fortunately she told me what I did and that we woldnever again make love. I cannot say all women are like this, but I had the good sense to never do it again. Yet, if you are caring, my experience is that you can leave a woman with both ovaries aching, and they can enjoy that feeling, and as lkong as you maintain that attitude, you will have many opportunities to explore this feeling with her again.
So, attitude is everything and if you give her the impression you will hurt her, you either won't get in in the first place or, if you do, you probably will hurt her. Hold her, engage in meaningful foreplay (that is, don't communicate the attitude that you just want the appetizer over with and you want to move on to the main course), take your time, tell her you care, be involved - and really mean it - and you can be one inch or fourteen (the two extremes I have seen myself) and you won't have this problem.