Back in the dark ages of V6 I developped a rule of thumb that the TEMP tablespace should be at least twice the size of the largest object in the database (table or index.) Depending on usage, it might have to be larger (and might be smaller), but that was the starting point. It was based on users doing full table sorts, etc., in a multiple user environment.
I still use it as a starting point, but it is less vital these days with sort areas, undo tablespace etc. and OLTP rather than DSS. Less vital, but causes less problems.
Joseph R.P. Maloney, CSP,CDP,CCP
'The answer is 42'