Frank's response is correct, but that does not include the Library space. The library space (1GB on the 4GB model) is totally separate, and is not able to be loaded with station music.
Frank is referring to the fact that if you don't load your 25 station model with 25 stations, the remaining free station space will be filled with the other stations over time. If you load it with only five stations, over time, all five will grow to be their maximum possible size... at least until you decide you want a 6th station, at which point it will shrink the oversized stations to make room for the new station. The 1GB of library space will always be there for you to add your own music to.
