Hello.
The only way to determine whether the size of your partner's penis would be uncomfortable is to try it. There's no "one-size-fits-all" rule here.
Here's some information that may be helpful:
1.Talk with your partner about the need for him to go very slowly and gently until you say he can begin to move.
2. Some people like to start with a clipped nail finger in the anus, moved very slowly inside. If there's pain, stop and wait a moment and breathe. This will allow the internal anal sphincter to relax. Once the discomfort stops, continue. If a finger is comfortable, you can proceed with other sex toys or a penis.
3. Unlike the vagina or the mouth, the anus and rectum don't have a built-in source of lubrication. In addition, the rectum is designed to absorb liquids so you've got to provide your own lube, and lots of it. The rule of thumb is to use as much as you think you need, and then use more. If you're using latex condoms, be sure to use one water-based (like Astroglide, Slippery Stuff, or Elbow Grease Gel) or silicone-based (like Eros's silicone lube), but it's important never to use petroleum-based or oil-based lubricants (like Vaseline or massage oil) since those destroy latex.
4. Although lubricant is a requirement, do NOT use any of creams (or any drugs) that dull nerves and mask any pain. These encourage tearing of the skin and other harm to the anus, since they dull pain. Remember that pain is our early-warning system and should never be ignored.
5. The key here is to go very slowly, and only proceed when you’re ready. In fact, you shouldn’t have penis-anus sex until you’re super turned on. Try changing your pattern. Instead of having p-a sex AFTER you have p-v sex and orgasms, try doing it before you orgasm, while you’re really turned on. If you do p-a sex before p-v sex, remember never to put anything in your vagina that’s first been in your anus because the bacteria that live in your anus can sometimes be hostile to those that live in your vagina. So if you want to back and forth, be sure your partner washes his penis/fingers thoroughly before inserting into your vagina.
6. And remember to follow risk reduction guidelines to avoid getting or giving infections, including HIV. For example, condoms need to be used for sexual activities where body fluids may be involved (including anal sex).
Finally, I highly recommend the book, “Anal Pleasure and Health,” by Jack Morin, Ph.D., widely available. This book is a very comprehensive resource for having fun with anal sex. Best of luck to you. Dr. J