First, ask them if you receive package too big to fit into your po box, will they hold it for you?
They should, they do it for me, and it's free, so if they will, it makes sense for you to get the smallest box.
Damn, I have a box thats $45 every 3 months. I just got the smallest box, even the largest wont hold but the very smallest package. They just keep it all behind the counter and put a note in my box.
It shouldn't matter what size, they should just slip a notification card in your box to get the package behind the counter. No package should be too big.
---PEACE---Mad Max