Action Details
Requirements and how-to articles for Google Ads's action:

paid action
This format is a mobile-optimised banner. These ads have been shown to work well as a large smartphone ad format, particularly when used at the bottom of a page. Suitable for mobile text and display ads.

You need this to get it done:

Mobile Leaderboard
You can create a responsive display ad, or upload a display ad. You can also choose to copy existing ads.
You can upload image ads as .gif, .jpg and .png
You can upload AMPHTML ads as a .zip containing 1 HTML document and up to 39 media assets.
Maximum file size: 150KB
Dimensions: 320 x 50 pixels
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Google AdSense Help - Guide to Ad Sizes
List of different ad sizes and how well each one performs.
Link missing here or found errors? Improve How-Tos