You cannot limit what TM uses on the drive. You can partition the drive and assign a specific partition as the TM backup drive. If you do this then be sure that partition is at least twice the capacity of the drive it backs up.
You don't pre-size a sparsebundle. It simply expands or contracts as needed. Given sufficient time TM will fill up any drive.
If your Drobo consists of RAIDed drives, then you cannot partition it. If it comes as two separate drives, then use one for backup.
If you're sticking w/the Drobo, listen to the Drobo people. As long as you're using the drive for TM, you've already listed your options. Want other options? Use another drive.
Basically, TM will use the space at hand, with one caveat. I prefer to follow the TM docs and use a drive that is larger than the drive(s) being backed up. This avoids the quicksand of filling up a TM and running into issues that presents. Using a bigger TM volume allows TM to handily delete oldest backups to make room for newer with juggling on your part.
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2. Time Tamer, a handy little script that does some magic and gives me a Sparse Bundle to use for Time Machine that is twice the size of my internal drive.
You might be able to change the maximum size of the sparse bundle, but it may not "stick." See #A8 in Time Machine - Troubleshooting.