Since 2006, some hard drives have 2.5 inches for use in servers. Since April 2008, is a 2.5 inches drive (with 15 mm height) with 3.5 inches marketed with the mounting frame as a desktop hard drive. This differs from USB to SPI.
1.8 inches drives since 2003 sub-notebooks, used in various industrial applications and large MP3 players. The width is 54 mm, the depth from 71 to 78.5 mm, height 8 mm. There are panels that have the overall width and height of 2.5 inches drives.
Even smaller sizes of 1.3 inches, 1 inches and 0.85 inches hardly play a role. The exception was Microdrives in the early days of digital photography, they enabled with a size of 1 inches wide and comparatively cheap memory cards CompactFlash Type II format for digital cameras, but were now replaced by flash memory.