Total size of a single mongod data set:

We're limited by the available virtual address space of the OS:

http://msdn.microsoft.com/en-us/library/windows/desktop/aa384271(v=vs.85).aspx[1]
http://unix.stackexchange.com/questions/61085/why-is-the-theoretical-ram-limit-for-rhel-6-128-tb-and-how-is-this-determined[2]

Linux 64TB with journaling (128TB without).
Windows 4TB with journaling (8TB without)