You will have to trial and error on the teat's size. Buy size #2 and if baby rejects then go #3. I dont think you will have to spoon feed your baby since your baby is just 1month. Keep trying on the sizes.
For me, my baby took #1 (for breast milk) very well at one month old but i prefer latching him on and have been doing so since. But because recently I started giving him water, he (coming 4months old) doesn't drink well with #1  so I spoon feed him and he is learning to drink little by little.