Not at the moment as tiles have to be a fixed width with the current grid system. I’ve noted the request though and will leave the discussion open so we can keep an eye on the level of community interest.
UPDATE! Scalable dashboards are now available (release notes)
We also have a feature request in for an optional ‘canvas’ style layout where you would define the aspect ratio of the canvas and then could position and size tiles relative to the canvas so the dashboard could scale and grow to fit various devices.
This wouldn’t necessarily solve the challenge of changing from landscape to portrait, but I think a ‘canvas’ layout would be a nice alternative for people who want more control over the relative position, size, and grouping of items on their dashboard. And it would be particularly helpful for people who want to show the same dashboard on different sized tablets (eg. 7", 8", or 10" tablet).