This is probably the best place to test truly weird breakpoints, it will scale to somewhere between mobile and large desktop. Click the button, give it a go.
Simply type a URL in the box above and hit enter, you can even test localhost.
These are pulled in from the global stats at statcounter, split by desktop and mobile/tablet
I've also added in lists of common devices, and common cruxes, to allow easy testing of nice and nasty conditions
Tip: For testing at screen sizes larger than yours, try zooming out to around 50%.