Dude, get your self an extension and you'll find out.
My ex-wife had a really deep vagina and lots of prior boyfriends with really big dicks. But to be real, dick size isn't on the top of the list in rating boyfriends for most women, it wasn't for her either, so I'd quit worrying about it. Gay guys, yeah dick size means a lot, most women, its not a deal killer. I know that's not true with most of the women that post here, but what would you expect, its a big dick forum.
But if you want to have some fun with her, get one of the extensions you'lll see on the "toy" sites, get one of the ones that has good reviews and sounds like it would fit your dick size. I used one on my ex-wife and it was a ton of fun. It you get one that fits right it acts a little like a cock ring, so even though you don't get as much stimulation you'll stay hard.
But even with reduced stimulation, watching my ex-wife go crazy from getting fucked with the big thing kept me way hard. Did the the big one feel better to her? Fuck yeah, but who gives a crap, pop that thing on and have some fun, why waste your life worrying about it. With this thing on, the total length was around 11" and I couldn't believe she could take the whole thing, with hard fucking it would just bottom out and she would really go nuts then.
I would use big dildos on her too and that would also drive her crazy, especially while she also used a vibe.
Also big fun was fucking her in her ass with my real dick while using a big dildo on her in her vagina.
So if you want to find out if she likes big ones, open the tool box and go to work, if she likes it just get some more toys and go for it, I had tons of fun doing this. If she wasn't a total psychopath I'd probably be doing it to her right now, instead of dinking around on this forum trying to find something exciting enough to jerk off over.