WC3 is an old RTS game so not designed for huge far distance values. Even a blank large map will cause World Edit to crawl when viewed from corner to corner (with no Z clipping).
That said do not lose hope of your idea. Not all needs to be as it seems, the wonderful world of perspectives and trickery.
If you are viewing from the ships perspective, you can change the size of the planet as you approach it to give a feeling of distance. Instead of having a very big planet very far away that will cause WC3 to crawl and have many problems, you could have a smaller planet a short distance away.
As your ship moves through space, you are physically not moving much but the planets change size to give the feeling that you have moved a long way. Since planets are round and if there is enough distance between, players should be none the wiser that the planets are the ones changing size and not that you are moving far.
So how to scale the planets? It is all based upon projection of an object onto a plane through a pinhole. The pinhole projection model. Instead of having the screen behind the pinhole, on computers you can have it in front so the graphics are the right way round. Once you have the ratios, the rest is much more simple. Obviously WC3 might use a different projection model, but it should be good enough for what you want to achieve.
In summary. Ship moves less and only when near planets. Planets change size when far away at a fixed distance.
