What size tampons should a 14 year old girl wear?

Tampons don't come in different sizes but in different absorbencies, your age doesn't effect what absorbency you should be using. You use the lowest absorbency for your flow and change absorbency throughout your period to match that flow: start with lowest absorbency, if they leak within 1-2 hours it's safe to use next absorbency up but if not saturated with blood after 4-6 hours use lower absorbency or switch back to pads if flow is too light for tampons.

At 14 years old tampons aren't recommended, it's best to use pads or menstrual cups, but if you do use tampons it's really important you understand the risks and precautions to follow to reduce risks.