These ships are generally too massive to land, so they're built in space and stay there in case they do see the interior of a planet's atmosphere, it's usually because something's gone horribly wrong - usually emphasized by the very agitated crew bracing for impact.

It may be a Starship Luxurious or so packed full of people or machinery that the interior resembles that of a submarine.

Now how do you get this across to people? By stating how big it is in miles or kilometers.