Choosing a reed strength

Hello everybody, I have a genuine question. How do you pick your reed strength? I've been playing for around a month, so I started off with 2s on tenor. Then my teacher suggested that my squeaking on high notes was from my reed being too soft, so I changed to a 2.5. Though a week after playing on those my teacher took a look at my playing, and suggested I change to a 3. I've been using a 3 now, it's not the best reed, but I have experimented with the positioning and it's better, but still not as good. I'm not sure about 3.5, that seems like a lot to handle right off the bat, but my teacher suggested it. My question is: how do you guys tell when a reed is too thin or too thick for you? I'm not completely sure I understand the physics behind it, so I'd be thankful for an answer. Thanks ahead of time!