THe stellar size in the planetsizes file corelates to the planet size line in the sector types file. You can make new ones, but you have to use the same word in both files.
The are some issues with creating new planets and random system creation you shoud be aware of. We covered it in another thread. It's a fairly long thread, but I recommend reading it.
http://www.shrapnelgames.com/cgi-bin...3;t=007597;p=1
Geoschmo