Currently, when using the designer, you can adjust the width of the canvas. It will snap to a few pixel widths specific to certain devices. But you cannot lock onto a width for a specific view mode

However, when you scroll from desktop views to tablet/phone views and return to desktop, the canvas width has returned to the full width of the available screen space. So if you set the canvas width to a specific value, you have to readjust the canvas width back. 

I recently started using an ultrawide 21:9 monitor and I find this frustrating because I want to view the canvas at 1920p wide, like most desktop screens. I wish I could pin the desktop view at a specific width.