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If your site contains any square size image you can use that size under media settings for post grid layout editor

View post on imgur.com

If you don’t see any square size image, then you can build it as following

 

#1, please add following code to your theme functions.php file

add_image_size( 'center-220', 220, 220, array( 'center', 'center' ) );

#2. install Regenerate Thumbnails plugins

https://wordpress.org/plugins/regenerate-thumbnails/

Then rebuild thumbnails,

View post on imgur.com

Now you can choose “center-220” on post grid layout editor

Hope this helps.

Regards