That feels like the wrong question to be asking. *Someone* will decide to enter data for stars a long way from inhabited systems. The useful set for any given player will vary a lot depending on where the play. Really, just get one full dump and then use the latest updatedate in the data for a date filter for the next pull, repeat. Yes, you're expected to pull and store data locally rather than always query EDSC on each use.
If you really want to make sure to pull all systems via a cubecoord filter then you'll want something like [-70000,70000],[-5000,5000],[-40000,100000]. Basically the galaxy is about 140k LY diameter, with the zero point being Sol, which is ~30k LY from the centre along the Z axis, and count at least 2000 LY up/down to get all of the reachable halo, but eh, call it 5k LY to be sure.