You could limit it with something like this:
rsync -azHK --delete-after /home/ /mnt/usbbackup/home/ |head -n 1000 >/backup-log/backup.txt
This will limit it to the 1000 first lines. Replace head with tail if you want the 1000 last lines, the last lines are probably more important. Personally I don't like email on successes at all, I just put the rsync line in cron, and I get emails on errors only.