Divine & Planar Powers

[Milo v3]

One of the Artefacts in my campaign (Which it is named after), has the ability to turn a mortal into a god. They also get one free plane for them to control.

What should I allow my players to do? Where should I draw the line?

Note: They haven't obtained the item (Or even heard of it) yet so I'm just planning ahead incase anyone every get ahold of it.

[Eldan]

An entire plane? Too much. Entirely too much, I'd say. That's infinite power, right there. I'd say that even a layer is a bit much. Gods get a divine realm, isn't that enough? Maybe give them an increased realm.

[Alleran]

Complete morphic control over an entire plane?

Hell (is that the right word here...?), you don't get that much power even if you become the Prince of Demons at the end of the Savage Tide adventure path. You only get morphic control over a single layer of the Abyss, and there's the slight downside of you eventually becoming the next Demogorgon (everybody rolls a 1 eventually).

Complete control over an entire plane is just asking for trouble unless you're in the high epic and already have divine ranks. At that point, it becomes a one-upmanship game anyway.

[Milo v3]

Its not the size of the material plane and its not infinite.

I was only going to give them 7686850 square kilometres (Basically the size of Australia which is just smaller than North America).

Edit: Also the artefact is the most powerful thing in all existance. Technically more than the gods. So yes it is overpowered. Thats kind of the point.

[Yora]

Depends of course on how many planes the setting has. When it's just 12 or 17 for the whole multiverse, it's severely over the scale.
If you have hundreds or even thousands, sure why not?

The size of Australia would be a sizeable, but still not very unusual demiplanes. Most settings have almost infinite numbers of those.

[Milo v3]

There are 20 current planes in my setting.

And Demi-planes are quite small in it. The largest being about the size of the state Victoria in Australia.

But from your replies I'm begining to think it might be to overpowered. So what limits should I place then?