The easiest way that I know of is for you to do a trial export with the prompt of filedata="BACKUPACTIVE" and "preview=YES"
The preview = yes will help you estimate the size and also let you know if there are any errors found on the tapes you need.
Ex:
export node node1 dom=unix filedata=BACKUPACTIVE dev=dev1 scratch=yes preview=YES fromdate=06/21/2006 fromtime=12:30:00
To actually see the outcome of this -
" q pr" will show the status
When done, check "q actlog" and scroll down until you see the export node completion info.
When ready to export for real, just swap in "preview=NO"
Good luck.