As I know the 2.5 inch is most used in laptop(notebook)
the PC use 3.5 inch generally,
Do you know the floppy disk, it's 3.5 inch ,but you can see the floppy disk dirver is a little big than the disk
its like 4x6 inches
I think it must be 3.5 inch.
to choose hard disk box the inter face is another most important feature!