I bought bigger sizes (2 and 3) for my 8 month old to try (we’ve just been using 0 and 1) and it seems to not really make a difference. She rarely gets a bottle anymore.
I was told to size up when feeds start to take longer. So if babe would drink a bottle and be done in 15 mins but now takes 30, size up. Every baby is different. Good luck :)
My 3 month old is still on a mix of size 0 and size 1 i tried a 2 but she was choking on it coming out too fast so clearly not ready for size 2 yet