Determining My screen size from preferences

Hi I am trying to determine what kind of screen size I have on my macbook pro from within the "about this computer" option in preferences.

I know it sounds dumb and could just measure with a ruler ( which I did ) but it came up as 14 inches and there is no macbook pro that's 14 inches it's either 15 or 13 inches

How can I determine my screensize from a preference option ?

thank you

Here's the macbook I have

Macbook pro laptop 2.4 HGz Intel Core 2 Duo running Mac Os 10.5.8


Thanks