Use the responsive design view built into firefox and try the different screen sizes. You still are going to care 'how it looks' on all of the devices you expe

Use the responsive design view built into firefox and try the different screen sizes. You still are going to care 'how it looks' on all of the devices you expect to see it. I understand that it's nearly impossible to control for every screen size, and maybe that's the point, but you should still know and understand what most users will see.