The Screen size is 90mm x 117mm (without status bar)
if 90/Scale=600 Pix then Scale=0.15
so 117/Scale=Height=117/0.15=780 Pix
the best Image size for the screen is ( 600 x 780 )Pix
Note: the screen aspect ratio(without the status bar) is the same of a Letter Page.
Link to the Original post
http://www.kuwaitvista.com/?p=498