The actual number of spread sizes depends on factors such as calendar work days, large differences in dates relative to the data date, ETL time duration, and the exclusion of zero value rows. Estimation size for SPREAD fact tables is more than 3 GB, which are all a part of the total tablespaces sizes.
A quick calculation uses a "best guess" on the average number of days for activities and resource assignments.
Total Activities: 1,000,000 X Average Activity Length: 5 = Total Spreads: 5,000,000