you can use the command;

mminfo -r "client,totalsize,name" -q "level=full,client= "

where client is name of client,
totalsize is size of saveset in bytes,
name is the name of saveset
is the name of client which you want to report of.

You can ommit client from -q part. So you will get all the clients' report

in this case I have to sum up manually to get total size of weekly full backup for each client.

you can also generate report all clients from management console.
Or you can send the result of mminfo into txt file and then get the total with a piece of code.