I just got my first plant and i was surprised at how bit it is. I ordered it from amazon and most people said theirs came in at about six inches.
Mine is about 12 inches with 9 leaves and two small sprouts on the bottom.
It looked very healthy so i transplanted it to a new pot as the roots were growing through the pot.

I will continue to monitor it.

T

Here is a link that might be useful: my photos

I have around a dozen banana varieties (~30 total) all in the ground, but I've starting pups in most kinds of soil including sandy soil with some composted soil mixed in (emulating my natural soil in the yard) an I have yet to kill one :-)

I'm sure you'll get better or at least different advice here, but in my experience I worry more about water and fertilizer than soils.