This tiny plugin adds the viewport meta tag with zooming permission to give your users the ability to zoom in your website with mobile browsers.
Viewport resizing comes to the WordPress admin bar
There is plugin switching the viewport of PC view and Mobile view.
A simple addition to the WordPress admin bar that displays the current viewport width.
Captures the viewport size of visitors' browser window and stores it as events in Google Analytics
A debugging tool that adds a small badge in the corner of the site, showing the current viewport width and height for finding media queries breakpoint