Normally, you can use any image size for your posts and Divi will automatically crop and resize the images to optimize your pages and perfectly align it in the Portfolio module. 

The ideal size for the Blog post items is 400x284px, but not less than that. This means it's  1:0.7 aspect ratio. You can use a bigger size for the images but make sure they have the correct aspect ratio. To calculate the correct size, you can use the following aspect ratio calculator:

If you want to disable the cropping system completely for the Portfolio module, you can add the following code to the functions.php file in your child theme folder.

function custom_portfolio_image_width($width) { return '9999'; }
function custom_portfolio_image_height($height) { return '9999'; }

add_filter('et_pb_portfolio_image_width', 'custom_portfolio_image_width' );
add_filter('et_pb_portfolio_image_height','custom_portfolio_image_height' );

If you don't have a child theme installed, please check refer to this tutorial.

If you don't see any changes, you might need to regenerate thumbnails, here is the plugin for this: https://wordpress.org/plugins/regenerate-thumbnails/

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