What would be the easiest way to automatically skip the backup of subfolders that use more space than a certain limit?
A solution that also sends a notification would get first prize!
If these are user folders, one solution is to set quotas on the folders and they won't go over your limit.
If it's a particular sub-type of data you should be able to either exclude an extension (such as for virtual disks) or exclude a particular subdirectory from the backup.
I'm wary of a solution that will just skip something because it's a certain size, as users tend to not understand why when a machine dies all their "stuff" didn't get restored. Seems simple enough but for consistency and simplicity usually a backup should be an all or nothing thing, especially with storage prices being what they are.