As Docchris says for the standard Drobo the limit is 2TB - so you can have mulitple 2TB volumes but not a single 16TB volume when using ext3.
The DroboDashboard will only give an ext3 option if you are formatting a Drobo connected to a DroboShare. This is the only supported configuration for using ext3 - other configurations such as direct attached to linux are in beta. So they work but Data robotics don’t give any direct support on it.
I’m not aware of anything really easy or reliable for reading an ext3 volume on Mac OS X. You may be better off doing it the other way round - formatting to HFS+ which can be read on both Mac and Linux.
One thing to be aware of is that Linux can only write to a Non-Journaled HFS+ volume. This means that if you are moving between OS X and Linux you need to make sure that it is cleanly ejected from OS X (so the journal is empty) then mount on Linux with “rw,force” options to ignore the journal.
Without a journal you need to be aware that if the Drobo is not unmounted properly on Linux (e.g. OS crash or power outage) then there is a greater potential for data loss and you definitely need to check the volume is consistent (fsck).