Thank you for the really awesome module. The module back-end has an option to configure the Avatar size in pixles. This setting adds height and width attributes to the <img> tag in the HTML.
The bug is that regardless of what you put in the field the actual image size is controlled by CSS instead. Thus anything you put in the backend is overriden by the CSS. Specifically:
modules/mod_recentdiscussions/assets/styles.css line 14. I deleted this line and the backend settings now work for me. Not sure if it is the correct way to fix it but...fyi. :-)