Imagine someone putting a battleship over a country, always in sight, in order to threaten them to not try anything funny. Imagine tech that allows to suspend the ship is not a problem.
How high is it possible to put a starship in the atmosphere in order for it to be visible clearly on sunny day? As a dark figure cast against clear sky.
What's the radius in which it will be visible, on the ground, assuming Earth is the planet?
What size should the ship be in order to be the size of moon from the surface?
UPD: To clarify. It's not the ship's size that is a problem. It's the atmosphere. It scatters light from the sun, otherwise the skies will be dark. But, this means if we place ship outside atmosphere, it will probably not be visible unless directly between the sun and observer.
Let's agree that the ship does not shine (is dark) and is visible because it blocks or absorbs light.