Ideally, the size of the platter ranges from 1.8" to 14" in diameter and largely depends on the physical size of the hard drive itself.
There are normally two basic physical sizes of hard drives such as 2.5-inch, used in the laptop computers, and 3.5-inch, used in desktop computers.
These, however, refer to the size of the disk platters and not on the size of the hard drive mechanism.