Hopefully, someone else will give exact details on the Z series... I'll say that on the B9180, at least, the sizes are stored in the registry.
If you open the registry editor and do a search for one of the sizes by name, that should help you find the location. If you have given your custom papers names that are somewhat unique (like "Baryta") that will make the search a little easier.
Once you have found the location of the papers, you will need to export that branch of the registry to save it out. You will want to save it somewhere that will be accessible after your re-install, of course. Be aware that the paper sizes may exist in more than one area of the registry.
After your re-install, you will need to import the branch back into the registry.
If all this sounds too complicated and you have no experience with the registry, then you may be better off just recreating the custom sizes after your re-installation.
With any luck, someone will pop in and say you don't need to do it this way on the Z!
Mark
