I’m afraid my boyfriend’s penis might be too big for me. Is this possible?

This is quite unlikely.

The vagina is a very elastic organ and is able to adjust its size to that of the erect penis.

You should prepare thoroughly for penetration by a large penis, however:

• Make sure foreplay lasts for a good while. Have your boyfriend initiate penetration only when you are fully aroused (excited), your vaginal walls are fully relaxed, and your vagina is well lubricated.
• Just before initiating penetration, add some artificial lubricant to facilitate the entrance of the penis.
• Communicate with your partner during sex. Let him know how you feel, how slow or fast you want him to go, and how deeply you want him to penetrate.
• Try to be as relaxed as possible. Tension will make your vaginal muscles contract, making penetration more difficult.

If your partner’s penis is unusually big and you’re afraid your vagina won’t be elastic enough to accommodate it, you might want to do some preparation on your own before your first time having sex with him:

• When you masturbate, once you’ve reached full sexual excitement, try inserting two fingers at the same time into your vagina. If this goes well, carefully add a third finger.
• Continue to masturbate with all three fingers, stretching the opening and the walls of the vagina with every movement.
• If you start feeling pain, go back to normal masturbation and finish as you normally do.
• After a few sessions of vaginal stretching, you’ll feel no pain and will be ready to move on to the real thing.
• If your problem is the length of your partner’s penis, and during penetration it feels like he is hitting the end of the vaginal canal, try avoiding positions that allow deep penetration, such doggy style or variations of the missionary position where you raise or bend your legs. 

If the problem persists, try using a sexual position that puts you in control and lets you choose the depth of penetration, like woman on top (cowgirl).