Right-click on the table in SSMS and go to the storage tab. You'll see the size of the table and the number of rows: from that you can calculate the approximate average size of a row. Now run a query that will return the number of rows that will be deleted, and multiply by that by the average row size to get the amount of data that will be affected. Don't forget to take indexes into account as well - you'll find the index size on the Storage tab.

John