For most smartphones the screen is 640*960 pixels.
When we say 'portrait' it's the standard term for a screen that's higher than it is wide (for example holding your phone normally).
When we say 'landscape' it's the standard term for a screen that's wider than it is high (for example holding your phone sideways).
The size of the screen won't change so on a regular phone portrait is 640*960 pixels and landscape is 960*640 pixels.