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That's not a cut and dried question. Your wallpaper size depends on a few things. first is the phone's screen, which in your case is 240 X 400 pixels. Then if you scroll the wallpaper, depending on the acceleration of the scroll, you'll have to expand it based on the number of home screens. I would begin with a photo at least 400 pixels high and 800 pixels wide. Then copy it to your phone, select menu>wallpaper and select the gallery for the location. then you can crop it appropriately to set it as wallpaper.