while there might be some other limitations in terms of drives and max sizes (such as not using drives which incorporate SMR features), the overall management of data as i understand it is as follows:
if drives are in a drobo, and data is stored on the drobo, then the data is spread over all disks automatically by the drobo (including data protection features for 1 drive, in SDR Single Drive Redundancy mode, or 2 drives in Dual DDR mode)
assuming SDR, then drobo will essentially use up the (total raw) capacity of your Largest drive for Redundancy.
(in your current case, then 6TB of raw capacity would be used up, though this would actually be spread out throughout the drives)
so assuming that you were to swap out a suitable 8TB compatible with your setup, then 8TB’s worth of capacity would end up being used up/reserved for protection.
if you have a moment to post back when you can, with your drive slot details if you wish, along with whether you have SDR or DDR currently enabled, we can try to flesh this out a bit more for you using your actual capacities.
Slot 1 = xTB drive (western digital WD xyz)
Slot 2 = xTB drive (etc)
etc or something similar