I asked this question a long time ago, but there is no answer out there. So I will answer my own question. Drobo has only tested up to 14TB hard drives in the 5C. There is no guarantee that larger drives will work. I finally had a pile of 18TB drives for another project so I decided to test it for myself.
Drobo Dashboard 3.6.1
Drobo Firmware 4.2.3
5x WD NAS Red Pro 18TB HDD
After some drives not mounting weirdness (I think we are at the very limit of the power supply), it mounted all 5 disks and let me format them. Because it’s so big, it forced me to make two 64TB volumes to access everything.
So far it seems to be performing well. I am running it in to see if it produces errors, random drive disappearances, or other instability, but so far it looks to work fine.
Write Throughput seems to max out at 75MB/sec.
Your mileage may vary. This is not tested by Drobo and is an unsupported configuration. Do so at your own risk.
I have some larger drives that I will test next.