Change gravatar image size

The gravatar image size (I’m using Titan theme) seems to be a lot smaller that on sites such as linked in; the image should be 80px x 80px but in comments section of my site it looks more like 40px x 40px. When i try and change the size in the style sheet it goes blurry. I’ve tried adding a size in the wp_list_comments in the comments.php but that didn’t work.

Anyone know how I can change this to the standard gravatar size without it going blurry?