Heather Corinna replies:
The vaginal canal in any woman isn't even one static size, anyway: it changes. When we're less aroused, the vaginal walls are tighter, and the back of the vagina and cervix are closer to the opening. When we become highly sexually aroused, the cervix pulls back, the back of the vagina "tents" and makes that canal longer, and the whole of the vagina and vaginal opening loosen.
I also hope you understand that different couples have different "fits" when it comes to their genitals, and that when it comes to penis-in-vagina intercourse, a male partner putting his penis into the vagina to the base of his penis isn't comfortable or always comfortable for every woman. For plenty of women with male partners with larger-than-average penises, a male partner putting his penis all the way in may NEVER be comfortable. Since angles are also an issue -- the vaginal canal is curved slightly upward, and erections can have all sorts of different curves of their own -- vaginal entry that's "all" the way in may be comfortable in some positions, or not others. Hopefully, before heading into intercourse, you'll both have other sexual experiences first so that you understand some of that already.
As well, usually for couples new to intercourse, even if everyone's size is a puzzle-piece perfect fit, for at least the first few times, deep vaginal entry probably won't feel so good, both because it's new, and because often everyone is so nervous and anxious that high arousal during first intercourse can be unlikely (so your vagina won't loosen and expand as much as it could otherwise). You and your partner may want to be a bit more gradual than that, depending on how it feels for you as you go.
There's nothing wrong with any of that, either, nor does something less than full-to-the-base entry not feel good for guys. Intercourse isn't any less "real" if any two people find that full penetration isn't comfortable for one of them.
In fact, it's pretty easy to figure out that what makes any kind of sex "real" and meaningful is when any two people care enough about each other's comfort and pleasure to make whatever adjustments and adaptations they need to so that it feels good for everyone, rather than trying to live up to some sort of ideal or external (and often unrealistic) standard.
Here are several extra links for you to round all of this out and drive home that this isn't anything to worry about. If the two of you have good communication together, both very much want to have intercourse and are emotionally and practically prepared for it, and you both are coming to it ready to be responsive to each other's needs and comfort, you've got all you need.