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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
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
Then rebuild thumbnails,
Now you can choose “center-220” on post grid layout editor
Hope this helps.