Hi
I've been looking into this too. My LOis just over 3 weeks and was on dr browns bottles with size 1 teats. He is a very hungry baby. However, in the past week I noticed he started to cry and fuss when feeding, taking an hour of perseverance to get him to finish a bottle. Sometimes it was impossible as he would just fall asleep and refuse. He then would wake up 30 mins later looking for more.
We have just moved to size 2 even though the recommendation for that dr brown teat is 12 weeks. He is dribbling quite a bit of milk out when feeding but that's getting better with each feed. He seems comfortable enough and is getting used to the change.
No harm in trying next size for a day or do and seeing how your LO does. Don't give up on it after 1 feedif he is dribbling or gulping is my advice, allow him to get used to it.
Joy x.