Just get the smallest one. If your package dosent fit they will throw it into a bigger locker that you can access with the key they will put in your small box
I have the smallest one too. But I don't get that kind of service; they leave a slip in the box saying it didn't fit & have to sign for it at the counter. Which for me, causes a slight inconvenience since I'm at work far away during postal counter hours. I end up having to wait until Saturday morning.