The dot pitch (pixel pitch) is not directly related to the overall size of the screen. A typical entry-level monitor has a dot pitch of 0.28 mm (measured diagonally). A good quality monitor has a dot pitch of 0.26 mm (measured diagonally).

I have two LCD monitors here, one an older HP 2335 (23") with a dot pitch of 0.258 and a ViewSonic VA2226W with a dot pitch of 0.282 mm. The HP blows the ViewSonic away.