Within Mobile Safari there are 16 different window sizes your page can be loaded into.
Here you can see a list of all these 16 sizes.

device width height orientation state image
iPhone/iPod 320 305 portrait initial frame, showing address bar & debug console
  320 356   initial frame, showing only the address bar
  320 416   scrolled down to hide the address bar
  320 460   fullscreen webapp
  320 480   fullscreen webapp with translucent statusbar
iPhone/iPod 480 158 landscape initial frame, showing address bar & debug console
  480 208   initial frame, showing only the address bar
  480 268   scrolled down to hide the address bar
  480 300   fullscreen webapp
  480 320   fullscreen webapp with translucent statusbar
iPad 768 946 portrait scrolled down to hide the address bar
  768 1004   fullscreen webapp
  768 1024   fullscreen webapp with translucent statusbar
iPad 1024 690 landscape scrolled down to hide the address bar
  1024 748   fullscreen webapp
  1024 768   fullscreen webapp with translucent statusbar