Use one of the load utilities to load the empty table with a representative sample of rows that constitutes a known percentage of the entire cardinality of the legacy table.
For the first iteration of testing, do not define any secondary indexes on the table. Analyze the space characteristics for this test table, add a secondary index, and repeat the task. Continue this process until you are reasonably certain that you have captured the characteristics of the base table and all the secondary indexes you initially want to define on it.
Extrapolate the total spool space required from an examination of the smaller temporary table. The scaled up data must provide a reliable picture of the initial space requirements for the table and its indexes.