You can find the true size of your hard disk by opening My Computer and right clicking on your hard disk (ie. click with the right mouse button). This will bring up a menu. Click on Properties (which is the last entry in the list). You should find the size of the hard disk displayed.
Note that this will be smaller than the size quoted when you bought the disk because the computer uses multiples of 1024 in calculating kilobytes, megabytes, and gigabytes whereas the manufacturers use multiples of 1000 so as to make the disk appear bigger than it really is. This gets more noticeable as disk drives get bigger. A one gigabyte drive would be reported by drive manufacturers as 1.073 gigabytes and make it look like the drive holds 73 megabytes more than it really does. A drive that the manufacturers quote as 80 gigabytes is really only 74.5 gigabytes.
If the reported size is still smaller than the size you expected even when you take this into account it still doesn't necessarily mean that you have been cheated as the unformatted drive size is always quoted by the drive manufacturer whereas once you format the drive some space is taken up by the file formatting and the operating system reports the resultant formatted size. You can't blame the drive manufacturers for this one because the formatted size varies depending on the file system that you use to format the drive and the drive manufacturer doesn't know which you will use. The formatted drive size should not be significantly smaller than the true unformatted size.
This article written by Stephen Chapman, Felgall Pty Ltd.