Any size limitation would be a function of the bridge chip in the external drive chassis, not in your Mac mini. 3TB external drives are common these days and 4TB external drives have been available since 2011. That size limit is based on the capacity of the available hard drives used in the enclosures, not on any technical limit on drive capacity.
As to partitioning your drive, my advice is do not partition. Back in the days of the OS X Public data, OS X 10.0, 10.1, and maybe 10.2 it was standard practice to partition drives and there were some technical limits imposed by OS X and the bridge chips used in the external enclosures. In fact in some cases not even partitioning would provide access to the full capacity of the larger drives. However, no matter what partitioning scheme I was able to come up with it was inevitable that a time would come when one partition would overflow and the diskspace I desparately needed was in the wrong partition. Technical limits have disappeared, or at least become so large (multiple petabytes or 1,000 terabytes) as to be inconsequential for now. As a result I no longer partition with the sole exception being the Recovery HD partition that Snow leopard and Mountain Lion automatically install on all boot drives.
