Recommendations on teats are always just suggestions. Depending on how strongly your child sucks and how strongly the teat presses against the roof of the mouth, different amounts of formula come out. It is best to let your baby decide which size and shape they prefer. As a good starting point: if you hold the bottle upside down, one drop per second should drip out.
With teats you learn from experience. If your baby needs a long time to finish a bottle, changing to a bigger teat might make sense. Ask for advice in a specialist shop. It is important that the teat suits your baby’s individual needs.