QuestionIf I do plan to have sex -- I don't plan on doing that for a while, but I don't know why I am asking this, but -- will I be able to satisfy my partner?
There are aspects of your question that I find very sad. It is not the question of curiosity or concern whether you will do well at sex. It is the underlying assumption that your first time at sex will be with someone to whom you are not married. For some reason you feel that if I knew you planned to wait until you were older that this would somehow make it better.
Sex is something God has given to married couples. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). The Greek word koite, literally refers to the act of sexual intercourse. We get our English word "coitus" from this Greek term. In marriage there is nothing wrong with a husband having sexual intercourse with his wife. In fact, in marriage it is considered pure. In contrast, outside of a marriage covenant, sex with someone to whom you are not married (fornication) or sex with someone who is married to someone else (adultery) are things God will judge. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10).
Our society has bought the lies of Satan and twisted the design of God all around. People have come to accept the false notion that sex comes first and then a pretense of marriage by living together and then, just maybe, marriage sometime later. It ought not to be this way. So much misery could be avoided if people would just listen to their Maker. I've known too many guys who think they can't marry a woman until they try her out. Yet it is those same guys who aren't married because they are more interested in the sex than a commitment. The saddest case was the guy who had a hard time remembering the names of the girls with whom he had intercourse. He could describe the circumstance, but he barely knew the girl because he had sex with them shortly after he met them and then soon lost interest. This isn't love! It is using women as objects.
When you buy into the falsehoods and start thinking that it is "normal," it is bound to happen eventually. All that is missing is an opportunity. "Woe to those who devise iniquity, and work out evil on their beds! At morning light they practice it, because it is in the power of their hand" (Micah 2:1).
So let's re-frame your question is a proper way, "Will I be able to satisfy my future wife when we have sex?"
Sexual satisfaction isn't about how big is your equipment. People have this tendency to assume that bigger is always better. There was a newspaper article last year that pointed out that bigger firetrucks aren't better in a city that already has narrow streets. Not too long ago we learned that bigger cars aren't necessarily better when gas prices double. Bigger schools don't necessarily mean a better education -- typically it means a worse one. People come in all sorts of sizes and yet men and women are able to marry and have great sex without any needing to check penis and vagina sizes. How is that?
A woman's vagina is much like a man's penis. It has a typical flaccid state and a sexually aroused state. In its normal state, it is about 3 to 4 inches long and about a half to one inch in circumference. So if you think just a moment you will realize that is smaller than most men's penises, which typically range from 3 to 8 inches long (the average being just over five inches) and a circumference of 4 to 6 inches (the average being 4.6 inches). It is only the outer third of a woman's vagina that has the majority of nerve endings that bring sexual pleasure, in other words it is only the first inch or so that matters to woman. On top of this, women find a penis bumping up against their cervix at the far end of their vagina to be painful. In other words, even a man with a penis far smaller than average is fully capable of giving a woman sexual pleasure. It is far more important how you engage in sex to give sexual satisfaction to your wife.
You might wonder how a guy's penis ever fits if a woman's vagina is so much smaller than a man's penis. It is simple really; the vagina contains a lot of folds which allows it to stretch to accommodate any penis size. It does mean that sex is uncomfortable the first few times for a woman as her body stretches to fit her husband's penis. The bigger the penis, the more stretching and discomfort she is going to experience at first. After a while, however, the wife's body begins to anticipate the size needed during sexual arousal and her vagina stretches and lengthens internally, matching the size of her husband's penis.
The vagina can adapt to a wide range of sizes, after all it is through this same vagina that a baby will eventually come. The average baby's head is about 12.5 to 15 inches in circumference! And yet afterwards the vagina will shrink back down to its normal size.
What this all means is that a married man doesn't have to worry about whether he is big enough to satisfy his wife. Within a month of having regular sex his wife's body will have adjusted to his particular size automatically. He ought to be more concerned about how he goes about having sex with his wife so that she always finds it an enjoyable experience. But bigger doesn't make sex better.