Well, limitless considering the amount of work put into it. I believe than you can build the super-bridge connecting multiple planets, or a death star (as they stated in their dev diaries) but the amount of resources you can obtain may be limited. I don't think the server is of unlimited size, at least not yet, so yes, on a certain scale you are limited. I believe they will expand the world as the need arises though, and so in a way it is endless.