The center of an OpenGL screen is 0.0f on the X and Y axis. To the left of center would be a negative number. To the right would be a positive number. Moving towards the top of the screen would be a positive number, moving to the bottom of the screen would be a negative number. Moving deeper into the screen is a negative number, moving towards the viewer would be a positive number.
"inches and millimeters - do these really have anything to do with OpenGL? The answer is, in a word, no. The projection and other transformations are inheritly unitless. If you want to think of the near and far clipping planes as located at 1.0 and 20.0 meters, inches, kilometers, or leagues, it's up to you. The only rule is that you have to use consistent unit of measurement."