Right click on Macintosh HD and click get info, then it says the capacity of your hard drive. (Unless you've got multiple partitions, then it's the size of that partition)
It is either printed on the bottom, Go to the hard drive>Applications>Utilities>Disk Utility or you can use Apple->About This Mac, then click on "More Info", then click on the ATA or Serial ATA bus, then click on the hard drive listed.