If you like to modify the size of the store thumbnails, then you can do this in two ways.
Change the size with PHP
If you like to change the size of the loaded images ( instead of scaling them with CSS ), then you can do this with the wpsl_thumb_size filter.
The code below shows how set the width and height of the images to 100. If you like to use a different size, then replace the 100, 100 with whatever you like, but do make sure you keep it within the maximum dimensions defined on the Media Settings page. Otherwise the images can look blurry.
The above code has to go in the functions.php in your active theme folder.
Scale it with CSS
Instead of changing the format of the loaded images through PHP, you could also use the CSS code below to scale the existing images.
But if you try to scale it to much, then the images may start to look blurry. When this happens, then it’s better to change the thumbnail size through PHP/
The CSS code has to go in the style(s).css in your active theme folder.