From MorphOS Library
PFSSetFileNameSize - Sets or retrieves the maximum length of filenames
VOLUME - The volume on which the filenamesize is to be changed or displayed. FILENAMESIZE - The new filenamesize. Must be between 31 and 107 and must be larger or equal the current size. If not supplied, the current filenamesize will be displayed.
WARNING: Use this command carefully, the filename length can only be increased, not reduced. The only way to set the filename length back to 31 after it has been increased is by reformatting the disk.