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

paid action
This format can fit into narrower spaces that don't accommodate the 160 x 600 wide skyscraper. The supply of available display ads for this ad size is more limited though, which may lead to sub-optimal ad performance. Suitable for text ads and display ads.

You need this to get it done:

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: 120 x 600 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