I use Resize Window & Viewport often when I'm developing websites as it helps me with responsive design. While it is quite simple, it does what I need which is just to quickly see how a page responds to different widths. It does that well. It's awesome!

This does exactly what it says and does it snappy and well. They execute their promise to perfection, so I give it five stars! I would love to have more width options, or the ability to add more, but those would be great features to add and do not reflect the delivery of what this extension promises. There is room to improve it, but

Working fine with no problems. The responsive mode (ctrl+shift+m) with draggable screen dimensions is very useful for checking all breakpoints.

Excelente

Great add-on, and the Ctrl+Shift+M mode really makes it shine (you should add this mode in the little graphical UI, given the amount of people that miss it)
Not given 6 stars because for really large sizes, it is a little buggy (selection areas go far right instead of centered above screen)
Really good job anyway

I would like to be able specify any width and height sizes as well as window position!