I'm right there with you on size, and I'm definitely not a virgin, so let me give you a bit of advice and commentary. First, you are definitely big. Our size is in the upper 5% of just about any of the surveys on size that you read. That's way above average. The numbers tell you that you ARE large. Are there bigger guys, sure. Don't let the preponderance of large dicks on this site make you think that there are lots of bigger guys, though. Think of it this way. If you score in the 95 percentile on a standardized test, you're not going to usually kick yourself because you're not in the 99 percentile. Where you are is more than adequate.
Second, here's a little info that should make you feel even better. I've heard on numerous occasions from lots of partners that our size is actually the best size from a receiving end. We're definitely big enough to satisfy and give a sense of fullness whether vaginal, oral, or anal is involved. But we're not so big that it's too difficult to accommodate us comfortably. As I've been told, "You have the perfect sized dick for...well, just about anything."
Third, regarding condoms, I'd recommend the Magnums, too. Back in the day when my wife and I used condoms, the larger sizes were not available. We used the lambskin ones, which made all the difference for us. I didn't realize it was the size of the regular latex ones that was such a killer on sensitivity. When I used the lambskin, there was actually some room for movement and it felt more natural. However, I don't recommend lambskin if you have multiple sexual partners or if your partner does either. They don't have the STD protection that the latex ones do. With the Magnums, the condom will fit like it's supposed to. I never felt like the latex were "uncomfortable" really, but they didn't fit well. Later, I discovered well fitting ones and it was like night and day.
Last, your questions about your gf and how she'll feel are good ones. Keep that mindset of trying to think about how your partner feels in sex, and seeking to please, and you'll become a skillful lover. You already have the equipment necessary, so now you'll need to get the experience and technique. It may sound weird to read a sex manual, but that's exactly what I did before I first had intercourse. Consequently, I went into the actual sex part with some good knowledge of not just mechanics, but also techniques.
I'm a lifelong learner type, and I still read up on sexual techniques and methods so I can satisfy my partner. Being informed and being attentive make all the difference. If it is the first time for both of you, take it slowly. Spend lots of time on foreplay. In fact, spend like twice or three times as long on foreplay as YOU think you need to. She'll appreciate it, and she'll enjoy it a lot more. Lots of kissing, body contact, oral and manual attention to her hot spots will get her ready for you. This is true even years later. The more she's lubricated naturally because she's into the love making, the better the experience.
Tell yourself to really enjoy the building up portion, and don't just focus on penetration. We guys tend to get one track minds and think it's all about "getting in." Well, that's definitley awesome, but enjoy the getting to getting in part and you'll both do better. Even still, she may be sore afterwards, and there may even be some bleeding.
Remind yourself and her that the first time is rarely great. It's cool cause it's a first, but it actually gets better. FYI, I've been married for 24 years, and sex is better now than it was as newlyweds. It was great then, but we can have lots more fun, spend more time, and just enjoy it more now because we both know more what we're doing. So don't set up expectations you can't meet.
Finally, remember that size is a cool thing to have. But, there is truth in the sayings about it being more about what you do with what you have. As a well hung man (and you are), you may not have to work as hard to impress or please, but you still have to make it a priority if you want to be a skilled lover. THAT, more than the size of your cock, is what impresses and pleases women.