I'm using size 2 tears for my little one, he is 9 weeks old. I'm not a massive fan of the tommee tippee bottles though, I've ordered some more MAM bottles x
We use mam Size 1 still because she's breastfed and apparently they prefer slower.
My little girl is 11 weeks on Wednesday and i changed all hers to size 2 on Sunday just because it seemed she was working hard to get the milk out faster resulting in her becoming even more messy, one of them she would always collapse it somehow and during the night i found sometimes she was literally having 2oz and falling asleep midway through