Here's a script I use for such purpose:
================8<=================
#!/usr/bin/bash
TOTAL=0
printf "\ntotal size of FULL backups for the last week run, by the groups:\n"
for NSRgroup in LIST OF YOUR GROUPS
do
  SIZE=`mminfo -ar totalsize -t'last week' -qlevel=full,group=$NSRgroup | awk\ 
{sum=sum+$0};END {print sum}'`
  SIZEgb=`expr $SIZE / 1073741824`
TOTAL=`expr $TOTAL + $SIZEgb`
  printf $NSRgroup":\t"$SIZEgb" GB\n"
done
printf "====\nTOTAL:\t\t$TOTAL GB\n\n"
=================8<==============

Obvoiusly, "mminfo" parameters can be modified

Cheers

+----------------------------------------------------------------------
|This was sent by dmitri_ryjikh AT ml DOT com via Backup Central.
|Forward SPAM to abuse AT backupcentral DOT com.
+----------------------------------------------------------------------

To sign off this list, send email to listserv AT listserv.temple DOT edu and 
type "signoff networker" in the body of the email. Please write to 
networker-request AT listserv.temple DOT edu if you have any problems with this 
list. You can access the archives at 
http://listserv.temple.edu/archives/networker.html or
via RSS at http://listserv.temple.edu/cgi-bin/wa?RSS&L=NETWORKER