sculpt: optionally limit the size for file-system expansion

[jschlatow]

When using a large USB stick for Sculpt, the expand operation can take a while. Instead of using the entire available storage space for the expansion, we could add a button to limit the expansion to, e.g., 4G so that the user has the option to either use all available disk space or to limit the disk space used by Sculpt to this fixed size.