Insecure about fianc�e's ex?

Ok first of all I have to say that I love my fianc�e very much and up until Saturday things were wonderful with us. Well I've always been a little insecure about my size um..down there. Well we were at a grocery store and getting some cucumbers and I joked around saying that's how big I wanted to be and she said " trust me you'd hurt me, I know this. " it caught me off guard and I asked her if she's had that big before and she said let's not talk about it and tried to change the subject. I started to dwell on it and I said who and after a bit she said her ex before me. I asked "so he was twice my size"? And she again stalled and said yes but then went on about how I was a good lover and it's not all about size and and that she loves me sonit doesn't matter. Well since then I don't really have a desire to sleep with her knowing this. I feel inadequate. Especially since early in our relationship i asked if I was her best and after she stalledahe admitted that he was but that he could never love her like I did. We're getting married in 4 weeks and I just feel like I'm cheating her.