if you are familiar with HTML, you can access the email notification code and see if you can adjust the size there...there may be a predefined size for the image being loaded like there are for logos.

Thanks, @lrnlab I did try that but was not able to. I am hoping someone else has cracked the code and can share how they were able to make the icons bigger. 

I tried a few options as well and could not find a solution...the short code unfortunately does not behave like an image file so not sure how that image is pulled in...pretty sure CSS would not help you here as the only option is what you can configure in the notification itself.