(1) the command you want depends on the details of your request.  do 
you want to know how many kbytes of data has been written to tape 
ABC123  if so, then you can use:
    /usr/openv/netbackup/bin/admincmd/bpmedialist -m ABC123 -U
  or
    /usr/openv/netbackup/bin/goodies/available_media

or do you want to see the size of each individual image that is 
written to tape ABC123?  that would be:

   /usr/openv/netbackup/bin/bpimmedia -mediaid ABC123 -L

(2) yes, every time a db backup occurs, the previous db backup 
written to that tape is overwritten.

HTH
rob

I'm trying to find out if there is a command that queries the size of 
images that is on the media used
for nbu database backups...Also, we have 2 tapes on rotation for db 
backup. Does it overwrite the previous
image with the new image? Any input is appreciated...Thanks in advance...