One of the big trends in web design is “responsive design”, where the layout of a website automatically adjusts to the size of a screen.
The good news is that, if you have Firefox on your computer, you already have a powerful tool to check how your website looks at different screen resolutions.
Just go to the website you want to check, and hit CTRL+Shift+M. You can also access it in the Tools menu, by clicking on Web Developer, and then Responsive Design View.
The pull-down menu at the top left allows you to choose from a range of common screen resolutions (circles below in red), while the scroll bar at the bottom right allows you to manually adjust the screen size (circled below in blue).