You could look at AWR reports and/or V$SQL_WORKAREA_ACTIVE to get an idea of what typical usage is, but of course, it only takes 1 rogue query to burn lots of temporary tablespace, so that easiest way is probably to make your temporary tablespace autoextend, so that it can grow to whatever is needed by your database.

Also, check out resumable operations which let you "pause" a statement when it runs out of temporary tablespace, rather than error. You can then (as DBA) react and decide whether you want to kill the query or allocate more space to the temporary tablespace.