There seems to be at least two problems when doing big scaling on objects within the editor. I tried scaling the default floor plane to 200x200x1 and encountered the following issues:
  1. Physics are messed up. Walking on the plane in Game mode with the default Player actor becomes very jerky, so collisions don't seem to work as expected. This is the most serious issue.
  2. Tiled textures start to look strange if they're tiled a lot, blurry and with jagged edges. This was more apparent when scaling to 2000x2000x1

If this is an inherent limitation of the engine that won't fix, you should at least alert and warn the user when they try to scale objects this big with a popup dialog or similar, and suggest alternative solutions.