Go to the Apple (top left corner): About This Mac: That window will tell you processor speed and how much RAM you have. Click the More Info... button. A new window should open up. There you will will find all your computers specs. There should be something like ATA or IDE that will be the connection for your iMac HD. It should say how large it is.
Or control and click the mouse button on the HD on the desktop and click get info. It should have the size in there.