Use 15" install DVD with 12" PowerBook I recently bought a 12" PowerBook G4 on eBay. I'd like to reinstall OS X to have a clean setup, but I can't find any original install discs online.

However, I found this one which says it's for 15" and 17" PowerBooks. Does that really matter? Could I use these disks with the 12" PowerBook?
Generally as long as you use an OS X build later than the one that came with the machine you should be good to go. In your case, it looks like all the 12 inch G4s can run 10.3, so most likely any retail 10.4 version should be fine.

For maximum safety, I'd go for 10.4.3 or later, as it seems that the last packaged version for a 12 inch G4 was 10.4.2 (build 8E90A), according to MacTracker.