Sizing estimates the amount of disk space needed to store the entire database.
Because the physical database is dependent on the underlying DBMS, I'll assume you're using Oracle8. These database objects' internal storage requirements are properly estimated in order to assign values to their storage components called initial and next extents . Each component maps onto its physical counterpart in the form of data files or storage bytes on disk files. A database consists of multiple tablespaces, which are logical concepts. Each tablespace spans one or more physical disk files that are assigned explicity to the tablespace itself.