If you format Drobo for a certain volume size (i.e. 2TB volume during initial format) and then add more protected capacity than can be contained in that volume (i.e. you add four 1.5TB drives over the course of a few days), Drobo Dashboard will prompt you to create a second volume for that new space. You can specify a size up to 16TB for that new volume (even if you don’t ever plan on having that much disk space).
Please note, Windows XP has a size limitation of 2TB.