My newborn is now a little over 2 months old. And she is wearing a mix of the Thirsties Duo wraps (with flats or 2+ preemie prefolds) and one-sized diapers. The first one-sized diaper we used was a Rumparooz. I stuff it with 1 Thirsties Duo Hemp size 1 insert, and it fits well. Since ours is snaps, it is sometimes hard to get a great stomach seal, but she never leaks. Then, we put a TotBots Easy Fit on her. I LOVE this on her, and we now have 2 of them to use. They are my go-to diaper when I have to change her outside the house. She also has 2 FuzziBunz OS in her stash. One is basically the new Elite diaper, and I love how the buttons are on the inside. These are stuffed with the Thirsties hemp or with the preemie prefolds/doublers too. Getting the 6 snaps on her isn't always easy, but it does fit GREAT on her.
At 2 months old, she was only 9.75 lbs and 24 inches long, so I am happy that these three brands fit her so well. We have a few other OS diapers in our stash, but I am not ready to move them to her just yet (as her brother is still in them). I MIGHT try the BabyKicks on her next though!