As of early 2015, the largest capacity hard drive being manufactured is 10 terabytes. Western Digital offers a hermetically sealed drive with helium inside to reduce friction and power use.
Currently, Sandisk manufactures the largest solid state hard drives at 4 terabytes each, though it hopes to manufacture SSDs as large as 8 terabytes by the end of 2015. There are no prices currently for these drives; they appear to be strictly for enterprise. The most common way to get larger amounts of storage space is to use multiple drives in what is referred to as a RAID (redundant array of independent disks).