I haven't created any large maps myself, but the largest one I have played was Chokepoint.  It has about 325 planets.  The thing that needs to be taken into consideration when designing a large map is how long it would take to conquer every single planet.  The map you're talking about, if I understand correctly, would have say 200 stars and 1000 planets at each star?  That means a total of 200,000 planets.  If you think how long it takes to attack, defeat, colonize, and rebuild a planet (10-20 min. usually)  you're looking at 50,000 hours (that's almost 6 years) of game play on that one map.

Maybe I misunderstood the size of the map you were talking about, but even a map with 1000 planets would be rather cumbersome without any real benefits.  I understand the urge to make REALLY large maps.  I just think taking a moment to think about the actual playability and travel time involved in simply moving a fleet to the other end of the map would be time well spent.

That being said, if you want to spend months on one map, that's perfectly alright.  I just don't like to spend more than 30 or 40 hours on one.