Enter a couple of numbers for your display, and it will tell you the width and height, and the DPI. Simple.

	Pixels wide	Pixels high
			Guess

Diagonal (" or cm)

Display name

Name	Pixels	DPI	Diagonal	Size	Pixel Count
19" wide	1440x900	89	19.0" (48.3 cm)	16.1" x 10.1" (40.9cm x 25.6cm)	1296000	Remove
19" 4:3	1280x1024	86	19.0" (48.3 cm)	14.8" x 11.9" (37.7cm x 30.1cm)	1310720	Remove
ibook	1024x768	106	12.1" (30.7 cm)	9.7" x 7.3" (24.6cm x 18.4cm)	786432	Remove