Can a user be banned on their host instance for breaking the host's rules on other instances?
If I go onto a different NSFW server with this user and post a 4K image of my phatt micropenis, could this user be banned on lemmy.ml for breaking the 'no porn' rule?
That would be up to the admin of your host instance. Hypothetically they can ban you for any, or even no reason. There's no software or rules preventing it.
So I would suggest making an account on the instance you want to post these photos to.
As I understand it, Lemmy is a place for discussing concepts under a general umbrella. Maybe make a c/micropenis to see if there's enough traction, then a server could make sense where you can set the rules. But those post won't be available across Lemmy if that's your desire. This being the case, and for anything nudity related, it'd be safe to assume there's plenty of space in the Fediverse for it, just not here.
How Phatt are we talking about here?
Diameter > length
Hell yeah.
What ratio do we think?
Anything less than 2:1 would be an exaggeration.