Open any file in the "standard" window size of your choice:
After typing in the URL or selecting a local file, click on the button of your choice. A new window will appear with your chosen file, opened at the specified size. This will give you an idea of how your pages will appear on the various sized computer monitors available to other users.
There is also a Simple Version and an Advanced Version of this Web Page Monitor Tester which present you with different options.