Folder size

When setting up backups I've got several attached USB drives. Unfortunately I need to think about whether the folders I'm backing up will fit on the available destination space. I'd like to be able to determine the total size of the folder I'm backing up and also understand the available storage on the destination. I understand the equivalent of du -sk can be costly so it should be an option to get this. Maybe add a "Properties" button to the list of option buttons that is displayed when I select a folder. Ideally if it could work like windows and the size property would update as it is computed. Or on the backup screen give me a "check capacity" button when setting up a new backup.