If you really want to go hard core and insane here on map sizes do this:
Play a single sector, largest size after having edited StarClusterSizeDefs.xml
To be instead say 800.
Instead of 90,000 tiles on the current ridiculous sized map it becomes a mind boggling 640,000 tiles. You'll never finish that game but it's pretty breathtaking.
The other thing is to run the game with this parameter: rebuilddatacheck which will cause the game to rebuild the changes you make to XML.
If you want to change the # of sectors the file you are looking for is StarClusterQuantifyDefs.xml
So on the biggest size you'd change it like this:
On the biggest size change many to say 20 to 30. Keep in mind, it'll start to flake out around 30.