change the overlay icon to image icon

Hi, i’ve used the code below to change the overlay icon that apears in all images:
.image-overlay.overlay-type-extern .image-overlay-inside:before {
background-image: url(//kriesi.at/wp-content/themes/kriesi/images/logo.png);
content: ”;
background-size: contain;
background-repeat: no-repeat;
background-position: 50% 50%;
background-color: transparent;
}

I need to change the size of my new image icon to be bigger, but i can’t find the css class to do that, can you help out?
Regards