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Trying Out Responsive Design in Looker Studio
Looker Studio's Responsive Design has been released, so I gave it a try right away. In conclusion, the experience is incredibly good and highly recommended.
You can create designs like the following:

Not available for existing reports
One point of caution. Since it's not available for existing reports, migration is required.
Supplement: Figure showing the failed attempt to make an existing report Responsive
Checking the Layout next to the Theme, I found the activation button for Responsive Design.

When I tried starting it up, it failed. Since it asked to create a new report, I decided to do so.

Creating a report with Responsive Design enabled
Creation procedure
When you create a new report, the Layout mode selection screen appears as follows. Selecting Responsive Design enables Responsive Design.

Once enabled, the screen looks like this. The author was excited to see a column-like layout appear.

Experiencing the Responsive Design report
Now that it's enabled, I've created a report.

When you narrow the width, it collapses into a single column!!!

You can select from several column sizes as follows. In previous versions of Looker Studio, you had to control graph sizes yourself, but from now on, it seems better to leave it to Looker Studio.

Adding rows is also easy—just click the button that appears. The height can be freely changed after creation.

Summary
Thank you, Google. This is great. If you haven't tried it yet, please try creating a new report.
I've also written about how to connect with Google Analytics, so please check it out if you're interested.
Discussion