The quickest way to recude the image size will be to write some CSS.
Find the CSS class(es)/id(s) of that icon, and in your theme’s style.css:
.icon-class{
width:20px;
}
If the image doesn’t go smaller, then you’ll need to set it’s width or max-width to 100%', and make it'sheightauto` whilst you’re at it.
If the image is actually an icon, then write the CSS rule accordingly.
The side-effect of doing it with CSS maybe that the image may become ‘blurry’, especially if it’s a png.
In this case, you’ll probably want to replace the actual image with a naturally smaller one….
In this case, you’ll have to put the image in your own theme’s directory and copy all the shortcode’s code into your theme (to avoid overriding if the third-party shortcode ever gets updated) and change the image’s path to your own image.