This applies to version 4.0 and later.
What can you expect in terms of how much disk usage os required when running PAS. This depends on two things. Are you storing events in the database? How many authentications are performed in a normal day,week or month.
Storing events in the database
By default events are store in the database and retained for 60 days. For the average installation this works fine.
200 000 authentications will claim about 50 MB of disk size. How fast you'll reach 200 000 authentications is variable to the installation.
In high transaction environments (more than 10000 events in 7 days) it is recommended not storing events in the database. The events should be offloaded to a siem solution.