Before making Google Ad Manager units responsive, you need to understand size mapping. To put simply, Size mapping maps the size of the browser (viewport) with the ads to be rendered to the user. We can size map via two predefined functions:
Here’s the overview:
Step #1: Generate Google Publisher Tags
Step #2: Create the ad size mapping
Step #3: Link the size mapping to GAM ad units.
Need more detailed instruction, head to our Google Ad Manager responsive ads blog post.