You are most likely running your computer's monitor at a low resolution — 1024 x 768 or below. If you'd like to be able to see more of your (and other) websites, you should go to the display settings/monitor resolution settings for your computer and make the screen resolution higher.
If you need the resolution to be low so that everything, especially text, appears as larger and easier to read, that's fine, but for insight into why the sites are the size they are, please see “My website is too small!” below.
Another good tip is that almost all modern browsers will allow you to zoom in, making text bigger, but still allowing you to run at a higher screen resolution. Check your browser's Help section for details.