What scale should I use for spaceships? The one I'm planning to draw is 5 km long, which, at Shipbucket scale, would be 33,333 pixels
You're going to be drawing a Star Destroyer?
Unless you're drawing real spaceships (of which currently there is none that would be too big for Shipbucket scale) you're better off using whatever arbitrary scale you think works best. We're not going to be uploading it to the database anyway since that would be potentially sticky from a copyright issue.