Hmm. I would think the Thirsties duo wrap size one snapped down to the smallest size should fit. What size are your prefolds? I used nb until about 10 lbs, then infant from 10-15 lbs, but plenty of people use infant pf's from birth on. (GMD brand sizes are different, though)
In general, sposies are much slimmer, so cloth can seem bulky, especially when they are so little, but like others have said, I also found it kept everything in better. If I had extra length I always folded it down in back to help form an extra poo barrier, then almost always used the jelly roll fold to keep stuff in.
And baby will grow faster than you will believe and you'll wonder how she ever fit in those tiny little things.
Christina, Mama to Salvador (3 yr) & Santiago (6 mo)! www.footprintsbaby.com Life has gotten too busy - Come shop our going out of business sale! Cloth Dipes & more.