If you ran the preupgrade, it scanned the system catalogs and calculates how much additional space you need to perform the upgrade.
If you did not run preupgrade, manually calculate how much free space to add to the system catalogs and databases. As a general rule, double the size of the largest catalog you are upgrading, and add approximately 10 percent more space. If you have a large number of stored procedures or compiled object in any database, the syscomments and sysprocedures catalogs will require additional space.