When to size up bottle teat

I use the same bottle brand and I understand what you mean. If your baby struggled to drink more milk, you can change to the nearest next stage teat. But if your baby is comfortable with the current teat, can finish milk without alot of milk dribbling out or choked when drinking, can continue with the teat. You can also try the next stage teat and observe your baby when he/she is drinking milk. So that your baby will not have worsened reflux. Some experimenting / trial & error needed but it'll all be alright 🙂 My baby was comfortable with size 1 teat for quite awhile. She is still using size 2 teat even now when she's already 1 yrs old as she prefers the teat's milk flow.