That crazy cyborg
I never did like the idea of full-scale planets in Starmade (I wasn't the one who suggested that), nor the idea of people mining them from orbit for this reason. I don't think orbital mining will be something that carries into the final game. There will probably be a better and more enjoyable way to mine planets and orbital salvage beam mining may not work. If the ideas I have outlined here were implemented, planets wouldn't be quite the full size of a sector because there would still need to be room for the atmosphere and planetary space above it. I would imagine they would spawn at different sizes and probably still be configurable, although making them too tiny might feel a little strange because the surface feels flat, and you wouldn't expect it to be. I don't think I'd set them to less than 1 km diameter.If you have Earth-sized planets, you might not be able to find/fight other players in a reasonable manner, and you might have way too many resources.
1KM diameter or so might be fine. And one gigantic planet near the core of each galaxy might be cool too.
Others would probably have more ideas why we don't want planets to be 10KM wide.