You should totally go for the 6.5mm, no one other than you and your fiance will really know what the carat size is, and if anyone asks you can say something like "just over a carat", etc., making it totally believable.
Remember depending on the setting it can make the stone look bigger or smaller. I think its a visual thing. I have size 7 fingers too, and I decided to go with a 6.5mm cushion. I think the 8mm and 9mm stones seem small on fingers, but if you think of ct 3.3 and 2.0 it seems huge. Go with what your heart says. lol. Thats how I picked out my custom setting. When I saw the sketch it made me cry. (or maybe that was b/c i was pmsing.)
I'm his little Ali-gator