Contrary to popular belief, skills and not size is the most important virtue when it comes to giving a girl orgasms.
The primary pleasure spot stimulated in a woman during penetrative intercourse is the G-spot, and it is located only 3 inches inside the vagina. Therefore, as long as you have a penis longer than 3 inches, you can easily rock her world. Some of the positions that may help you are:
Doggy Style: Your partner bending on all fours, and you penetrate her from behind. A classic position that most people are familiar with. Apart from being extremely pleasurable for the man entering the woman, the doggy style puts the lady in the ideal posture for the penis to reach the G-spot. With a little positioning, you can reach the G-spot (and hit it repeatedly) when you are doing it doggy-style.
Missionary Grind & Swivel: Remember that the key to stimulating her during penetration is not moving your penis in and out, but rubbing it in and around her vagina (and G-spot). Enter her in the missionary position, and grind your penis all inside her while maintaining the in-and-out motion. The idea is to provide maximum rubbing stimulation inside her vagina and onto the G-spot.
Girl on Top: Apart from hitting the G-spot in the right manner, this position also allows your partner to be in control. This, in turn, gives her the authority and movement to maximize her pleasure all by herself. Many experienced women know what they want, and letting them have control will make the job much easier for you!
FInding the right spots and positions has a bit of a learning curve, but you will surely get there as long as you do not give up!