Most modern web sites are made to be completely viewable on a monitor with a screen that can display 800x600 pixles. At least if the designer has half a brain! some newer websites are made for 1024x768 and yet others are made for more modern higher res monitors that can display 1152x864 or 1280x 1054. A lot depends on the designer and what they feel is the standard or how smart they are or not.
You can change the resolution on your monitor in the control panels. some resolutions may not be available on your system depending on your video card and monitor.
Today most monitors that I am aware of are at minimum 1024x768.
Here is a good site that will explain it better than I can:
screen resolutions