Ok so female perspective... the first time I fucked a really big dick in front of my husband I was a pretty nervous because I was concerned how he would feel. So initially I held back in showing how good I was feeling, it was hard to get out of my own head. Now with me I always feel a little bit of guilt until I have my first orgasm, then its like a switch gets flicked in my brain and I go oh yeah this is fun, then I'm fine and I lose my jitters and dive in. So when the first orgasm hit I said screw it and fucked him the way I wanted to be fucked. In the end my husband was fine, he said at first he was surprised as he thought I'd go nuts on it, he was glad when I finally did.
My advise is don't worry so much, if your nervous she will like it too much she is nervous you will be jealous or upset. Sex is fun, so have fun. Big dicks are a lot of fun, but connecting to a man and having sex is a lot of fun too. Just because she enjoys one big dick doesn't mean she won't like your cock any more. I have fucked plenty of medium to small dicks and I always found a way to have fun.