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

paid action
This format can fit into smaller spaces that don't accommodate the larger rectangles. The supply of available display ads for this ad size is generally limited though, which may lead to sub-optimal ad performance. Suitable for text ads, display ads and mobile text and display ads.

Requirements
You need this to get it done:

Square
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: 250 x 250 pixels
Requirements missing or found errors? Report

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

0
Google AdSense Help - Guide to Ad Sizes
List of different ad sizes and how well each one performs.
Official
Link missing here or found errors? Improve How-Tos