Hey brother...
Don't sweat it... It's not about if 'you' can get her off.. It's really if 'she' wants you to get her off. Some women are attracted to more well endowed men, just as some men are more attracted to slimmer women or larger breasts. Everyone has preferences... The truth is, if you're attracted to the person you're with, their physical attributes will always 'get you off' regardless of penis size, breast size, weight, height, etc.
There will be some girls that won't get off on it, as there are some girls that are disgusted/pained by a man that is too well-endowed.
The trick to this is, to find a girl that suits your attributes.. (i don't only mean physically, but emotionally as well.) likewise, she should fulfill your needs in a relationship partner.
Majority of men are 6".. Meaning there are quite a few that are smaller as well... We're talking fully grown men here as well.
What you see in pornography is not a reflection of reality. Just like the women's bodies are perfect, they all like to be cummed on their face or in their mouth, etc... It's not reality. In reality not every girl has such an amazing body, or perfectly full breasts or nipples. Likewise, not every guy is muscular and well endowed. So don't base your personal judgements on what you see in pornographic images and videos.
Be safe,
justin