Without seeing your code it's hard to diagnose your problem.
Are you profucing a 2D or 3D display ?
For 2D the screen is always the same size, -1 < X < +1 and -1 < Y +1. You can get the size in pixels from graphics_get_display_size() and work out the appropriate scaling factors to use in your vertex shader.
For 3D it will depend on any transforms your vertex shader performs.
PS: You really shouldn't be thinking about "pixels" in openGLES