You can use the already mentioned built in Responsive Design Mode (via dev tools) for setting customised screen sizes together with the Random Agent Spoofer Plugin to modify your headers to simulate you are using Mobile, Tablet etc. Many websites specify their content according to these identified headers.
As dev tools you can use the built in Developer Tools (Ctrl + Shift + I or Cmd + Shift + I for Mac) which have become quite similar to Chrome dev tools by now.