Hi vizou,
To control the size of the image on the store page, you’ll go into your Exchange file and into /lib/templates/content-store/elements. There you’ll find and copy the featured-image.php file. In your copy of the file, on about line 20, you’ll find where the image size is set.
<?php it_exchange( 'product', 'featured-image', array( 'size' => 'large' ) ); ?>
Try changing ‘large’ to ‘thumbnail’ (it uses the default WordPress image sizes). In your WordPress dashboard, go to Settings > Media and set your Thumbnail size to whatever you’d like it to be. Then you’ll create a directory in your child theme: /wp-content/themes/YourTheme/exchange/content-store/elements/. There, you’ll upload your copy of the featured-image.php file. That should take care of your images all being the same size on your store page.
If you changed your image sizes in Settings > Media, it won’t change the size of images already on your site. You might look into plugins like this to regenerate the older images on your site: https://www.ads-software.com/plugins/regenerate-thumbnails/
Let us know if you have any additional questions.
Thanks,
Elise