Page visibility is influenced by the resolution of the user's screen. The standard resolution values are 640x480, 800x600, 1024x768, 1280x1024, 1400x1050 and 1600x1200 pixels (picture elements = pixels).

Designers creating web pages usually work on high resolution screens.

Most users have 600x800 pixel resolution screens. Besides, search engines take up a lot of space as well. It is impossible to know which search engine the user has or how it is set. That is the reason why web pages should be designed in such a way that they display the most important part of the content even on the lowest resolution screens.

One example of an excellently designed page fully visible on screen resolutions from 600X800 to 1024X768 pixels is the cad lab portal at the Faculty of Mechanical Engineering and Naval Architecture.  
 
It is important not to place the important navigation at the bottom or right end of the page, since it will be covered on the lowest visibility screens.
 
When a 600 pixel page is opened on a higher resolution screen, it opens along the left end of the screen, leaving the right end empty. It is a lot nicer visually when a page is automatically placed in the middle of the screen.

The page visibility with different resolutions can be tested on the following web pages:
Browsersizer
Browsermaster

Links to examples we used:

cad lab portal (excellently designed page visible on screen resolutions from 800X600 to 1024X768) Browsersizer (page visibility testing with different resolutions) Browsermaster (page visibility testing with different resolutions)