100 systems/15 planets sounds a lot like the SE4 *defaults*.
Fun for an average game; not too large not too small.
In case you don't know, all the modders know that SE4 will operate just fine with 250 system maps, but not with very much more.
Now, consider the first entries in the list at CK's site:
http://www.captainkwok.net/se5info.php
Also note the following very basic computer facts:
- Computers store numbers in multiples of one Byte.
- One Byte can store the whole numbers zero through 255
- Two Bytes can store the whole numbers zero through 65,535
- Three Bytes is almost never used
- Four Bytes can store the whole numbers zero through 4,294,967,295
Now, one of those numbers probably looks familiar. One of the numbers is damn big. And the other is unfathomably huge.